Remove SystemLayerImplLibevent

[kpschoedel]

Problem

The libevent-based implementation of System::Layer is unused, untested
in CI, and probably stale. (It was originally written as a sanity check on
System::Layer APIs, chosen as a representative substrate independent of
any active platform.)

Change overview

Remove SystemLayerImplLibevent.

Testing

CI to confirm build edits.

[github-actions]

PR #19396: Size comparison from ac7e2f7 to 9484912

Increases (4 builds for cc13x2_26x2, esp32)

platform	target	config	section	ac7e2f7	9484912	change	% change
cc13x2_26x2	lock-ftd	LP_CC2652R7	(read only)	683455	683463	8	0.0
			.text	584788	584796	8	0.0
	pump-app	LP_CC2652R7	(read only)	664899	664907	8	0.0
			.text	578052	578060	8	0.0
	pump-controller-app	LP_CC2652R7	(read only)	655483	655491	8	0.0
			.text	570748	570756	8	0.0
esp32	all-clusters-app	c3devkit	(read only)	1007740	1007742	2	0.0
			.flash.text	1007740	1007742	2	0.0

Decreases (4 builds for cc13x2_26x2, telink)

platform	target	config	section	ac7e2f7	9484912	change	% change
cc13x2_26x2	lock-ftd	LP_CC2652R7	(read/write)	159376	159368	-8	-0.0
	pump-app	LP_CC2652R7	(read/write)	178844	178836	-8	-0.0
	pump-controller-app	LP_CC2652R7	(read/write)	188364	188356	-8	-0.0
telink	lighting-app	tlsr9518adk80d	(read/write)	801716	801708	-8	-0.0
			text	569298	569296	-2	-0.0

Full report (25 builds for cc13x2_26x2, cyw30739, esp32, k32w, linux, mbed, nrfconnect, p6, telink)

platform	target	config	section	ac7e2f7	9484912	change	% change
cc13x2_26x2	all-clusters-app	LP_CC2652R7	(read only)	689943	689943	0	0.0
			(read/write)	161824	161824	0	0.0
			.bss	74660	74660	0	0.0
			.data	3392	3392	0	0.0
			.rodata	112327	112327	0	0.0
			.text	577304	577304	0	0.0
	all-clusters-minimal-app	LP_CC2652R7	(read only)	640599	640599	0	0.0
			(read/write)	158132	158132	0	0.0
			.bss	73884	73884	0	0.0
			.data	3332	3332	0	0.0
			.rodata	89367	89367	0	0.0
			.text	550912	550912	0	0.0
	lock-ftd	LP_CC2652R7	(read only)	683455	683463	8	0.0
			(read/write)	159376	159368	-8	-0.0
			.bss	72612	72612	0	0.0
			.data	3256	3256	0	0.0
			.rodata	98183	98183	0	0.0
			.text	584788	584796	8	0.0
	lock-mtd	LP_CC2652R7	(read only)	632879	632879	0	0.0
			(read/write)	145720	145720	0	0.0
			.bss	68348	68348	0	0.0
			.data	3256	3256	0	0.0
			.rodata	98071	98071	0	0.0
			.text	534316	534316	0	0.0
	pump-app	LP_CC2652R7	(read only)	664899	664907	8	0.0
			(read/write)	178844	178836	-8	-0.0
			.bss	72756	72756	0	0.0
			.data	3292	3292	0	0.0
			.rodata	86363	86363	0	0.0
			.text	578052	578060	8	0.0
	pump-controller-app	LP_CC2652R7	(read only)	655483	655491	8	0.0
			(read/write)	188364	188356	-8	-0.0
			.bss	72860	72860	0	0.0
			.data	3252	3252	0	0.0
			.rodata	84251	84251	0	0.0
			.text	570748	570756	8	0.0
	shell	LP_CC2652R7	(read only)	682230	682230	0	0.0
			(read/write)	165032	165032	0	0.0
			.bss	76956	76956	0	0.0
			.data	3396	3396	0	0.0
			.rodata	108926	108926	0	0.0
			.text	572992	572992	0	0.0
cyw30739	light	cyw930739m2evb_01	(read/write)	602562	602562	0	0.0
			.app_xip_area	461508	461508	0	0.0
			.bss	84008	84008	0	0.0
			.data	732	732	0	0.0
			.rodata	0	0	0	0.0
			.text	112	112	0	0.0
	lock	cyw930739m2evb_01	(read/write)	599646	599646	0	0.0
			.app_xip_area	458456	458456	0	0.0
			.bss	84176	84176	0	0.0
			.data	700	700	0	0.0
			.rodata	0	0	0	0.0
			.text	112	112	0	0.0
	ota-requestor-no-progress-logging	cyw930739m2evb_01	(read/write)	599446	599446	0	0.0
			.app_xip_area	459372	459372	0	0.0
			.bss	83140	83140	0	0.0
			.data	616	616	0	0.0
			.rodata	0	0	0	0.0
			.text	112	112	0	0.0
esp32	all-clusters-app	c3devkit	(read only)	1007740	1007742	2	0.0
			(read/write)	1481834	1481834	0	0.0
			.dram0.bss	69168	69168	0	0.0
			.dram0.data	14656	14656	0	0.0
			.flash.rodata	212584	212584	0	0.0
			.flash.text	1007740	1007742	2	0.0
			.iram0.text	62902	62902	0	0.0
		m5stack	(read only)	1062491	1062491	0	0.0
			(read/write)	483968	483968	0	0.0
			.dram0.bss	74688	74688	0	0.0
			.dram0.data	34200	34200	0	0.0
			.flash.rodata	243084	243084	0	0.0
			.flash.text	1057107	1057107	0	0.0
			.iram0.text	123267	123267	0	0.0
k32w	light	k32w061+release	(read/write)	653784	653784	0	0.0
			.bss	70044	70044	0	0.0
			.data	2004	2004	0	0.0
			.text	575936	575936	0	0.0
	lock	k32w061+release	(read/write)	714876	714876	0	0.0
			.bss	70484	70484	0	0.0
			.data	1976	1976	0	0.0
			.text	636616	636616	0	0.0
linux	chip-tool-no-interactive-ipv6only	arm64	(read only)	9502940	9502940	0	0.0
			(read/write)	678033	678033	0	0.0
			.bss	43681	43681	0	0.0
			.data	1152	1152	0	0.0
			.data.rel.ro	614464	614464	0	0.0
			.dynamic	528	528	0	0.0
			.got	14936	14936	0	0.0
			.init	24	24	0	0.0
			.init_array	192	192	0	0.0
			.rodata	467916	467916	0	0.0
			.text	7486692	7486692	0	0.0
	thermostat-no-ble	arm64	(read only)	2544060	2544060	0	0.0
			(read/write)	183057	183057	0	0.0
			.bss	91409	91409	0	0.0
			.data	1512	1512	0	0.0
			.data.rel.ro	82120	82120	0	0.0
			.dynamic	528	528	0	0.0
			.got	4992	4992	0	0.0
			.init	24	24	0	0.0
			.init_array	400	400	0	0.0
			.rodata	160164	160164	0	0.0
			.text	2147040	2147040	0	0.0
mbed	lock-app	CY8CPROTO_062_4343W+release	(read only)	6224	6224	0	0.0
			(read/write)	2429656	2429656	0	0.0
			.bss	202692	202692	0	0.0
			.data	5872	5872	0	0.0
			.text	1392300	1392300	0	0.0
nrfconnect	all-clusters-app	nrf52840dk_nrf52840	(read/write)	1192051	1192051	0	0.0
			bss	141362	141362	0	0.0
			rodata	154644	154644	0	0.0
			text	817112	817112	0	0.0
	all-clusters-minimal-app	nrf52840dk_nrf52840	(read/write)	1139775	1139775	0	0.0
			bss	140579	140579	0	0.0
			rodata	131136	131136	0	0.0
			text	789188	789188	0	0.0
p6	all-clusters-app	default	(read/write)	2544632	2544632	0	0.0
			.bss	137120	137120	0	0.0
			.data	2808	2808	0	0.0
			.text	1502896	1502896	0	0.0
	all-clusters-minimal-app	default	(read/write)	2489440	2489440	0	0.0
			.bss	136328	136328	0	0.0
			.data	2752	2752	0	0.0
			.text	1447704	1447704	0	0.0
	light-app	default	(read/write)	2421312	2421312	0	0.0
			.bss	129432	129432	0	0.0
			.data	2600	2600	0	0.0
			.text	1379576	1379576	0	0.0
	lock-app	default	(read/write)	2441648	2441648	0	0.0
			.bss	129256	129256	0	0.0
			.data	2576	2576	0	0.0
			.text	1399912	1399912	0	0.0
telink	light-switch-app	tlsr9518adk80d	(read/write)	781696	781696	0	0.0
			bss	70636	70636	0	0.0
			noinit	40416	40416	0	0.0
			text	552576	552576	0	0.0
	lighting-app	tlsr9518adk80d	(read/write)	801716	801708	-8	-0.0
			bss	70888	70888	0	0.0
			noinit	40416	40416	0	0.0
			text	569298	569296	-2	-0.0