Remove WriteClientHandle leftovers (followup of #13248)

[vivien-apple]

Problem

After #13248 WriteClientHandle is gone. I was using it locally and when updating it was complaining about the fact that the class was incomplete due to the forward declaration.

Change overview

• Remove the leftovers...

[github-actions]

PR #13774: Size comparison from fa0b36f to 579fe98

Full report (32 builds for efr32, esp32, k32w, linux, mbed, nrfconnect, p6, qpg, telink)

platform	target	config	section	fa0b36f	579fe98	change	% change
efr32	lighting-app	BRD4161A	(read only)	832748	832748	0	0.0
			(read/write)	127032	127032	0	0.0
			.bss	125136	125136	0	0.0
			.data	1896	1896	0	0.0
			.text	832740	832740	0	0.0
		BRD4161A+rpc	(read only)	820128	820128	0	0.0
			(read/write)	143696	143696	0	0.0
			.bss	141696	141696	0	0.0
			.data	1996	1996	0	0.0
			.text	820120	820120	0	0.0
	window-app	BRD4161A	(read only)	803316	803316	0	0.0
			(read/write)	125720	125720	0	0.0
			.bss	123872	123872	0	0.0
			.data	1848	1848	0	0.0
			.text	803308	803308	0	0.0
esp32	all-clusters-app	c3devkit	(read only)	916434	916434	0	0.0
			(read/write)	1317506	1317506	0	0.0
			.dram0.bss	70720	70720	0	0.0
			.dram0.data	14244	14244	0	0.0
			.flash.rodata	178952	178952	0	0.0
			.flash.text	916434	916434	0	0.0
			.iram0.text	62056	62056	0	0.0
		m5stack	(read only)	965003	965003	0	0.0
			(read/write)	449464	449464	0	0.0
			.dram0.bss	75184	75184	0	0.0
			.dram0.data	34032	34032	0	0.0
			.flash.rodata	208120	208120	0	0.0
			.flash.text	959619	959619	0	0.0
			.iram0.text	123399	123399	0	0.0
k32w	light	k32w061+release	(read/write)	658572	658572	0	0.0
			.bss	76584	76584	0	0.0
			.data	1864	1864	0	0.0
			.text	574324	574324	0	0.0
	lock	k32w061+release	(read/write)	659368	659368	0	0.0
			.bss	76824	76824	0	0.0
			.data	1884	1884	0	0.0
			.text	574860	574860	0	0.0
linux	chip-tool-ipv6only	arm64	(read only)	8377644	8377644	0	0.0
			(read/write)	386113	386113	0	0.0
			.bss	56049	56049	0	0.0
			.data	1128	1128	0	0.0
			.data.rel.ro	257200	257200	0	0.0
			.dynamic	560	560	0	0.0
			.got	67992	67992	0	0.0
			.init	24	24	0	0.0
			.init_array	200	200	0	0.0
			.rodata	429972	429972	0	0.0
			.text	7119508	7119508	0	0.0
	thermostat-no-ble	arm64	(read only)	2040876	2040876	0	0.0
			(read/write)	145393	145393	0	0.0
			.bss	64753	64753	0	0.0
			.data	904	904	0	0.0
			.data.rel.ro	72728	72728	0	0.0
			.dynamic	560	560	0	0.0
			.got	4064	4064	0	0.0
			.init	24	24	0	0.0
			.init_array	312	312	0	0.0
			.rodata	130028	130028	0	0.0
			.text	1696688	1696688	0	0.0
mbed	all-clusters-app	CY8CPROTO_062_4343W+release	(read only)	6224	6224	0	0.0
			(read/write)	2349632	2349632	0	0.0
			.bss	189428	189428	0	0.0
			.data	5296	5296	0	0.0
			.text	1312208	1312208	0	0.0
	lighting-app	CY8CPROTO_062_4343W+release	(read only)	6224	6224	0	0.0
			(read/write)	2332328	2332328	0	0.0
			.bss	180936	180936	0	0.0
			.data	5576	5576	0	0.0
			.text	1294928	1294928	0	0.0
	lock-app	CY8CPROTO_062_4343W+release	(read only)	6224	6224	0	0.0
			(read/write)	2302744	2302744	0	0.0
			.bss	179936	179936	0	0.0
			.data	5560	5560	0	0.0
			.text	1265344	1265344	0	0.0
	pigweed-app	CY8CPROTO_062_4343W+release	(read only)	6224	6224	0	0.0
			(read/write)	1139712	1139712	0	0.0
			.bss	11756	11756	0	0.0
			.data	4368	4368	0	0.0
			.text	103096	103096	0	0.0
	shell	CY8CPROTO_062_4343W+release	(read only)	6224	6224	0	0.0
			(read/write)	2054256	2054256	0	0.0
			.bss	156876	156876	0	0.0
			.data	4864	4864	0	0.0
			.text	1016856	1016856	0	0.0
nrfconnect	lighting-app	nrf52840dk_nrf52840	(read/write)	940935	940935	0	0.0
			bss	119020	119020	0	0.0
			rodata	108492	108492	0	0.0
			text	635832	635832	0	0.0
		nrf52840dk_nrf52840+rpc	(read/write)	926415	926415	0	0.0
			bss	116064	116064	0	0.0
			rodata	100940	100940	0	0.0
			text	631228	631228	0	0.0
		nrf52840dongle_nrf52840	(read/write)	991611	991611	0	0.0
			bss	121864	121864	0	0.0
			rodata	113244	113244	0	0.0
			text	668032	668032	0	0.0
		nrf5340dk_nrf5340_cpuapp	(read/write)	850762	850762	0	0.0
			bss	115808	115808	0	0.0
			rodata	101668	101668	0	0.0
			text	552756	552756	0	0.0
	lock-app	nrf52840dk_nrf52840	(read/write)	910295	910295	0	0.0
			bss	118176	118176	0	0.0
			rodata	103460	103460	0	0.0
			text	611280	611280	0	0.0
		nrf5340dk_nrf5340_cpuapp	(read/write)	820346	820346	0	0.0
			bss	114992	114992	0	0.0
			rodata	96684	96684	0	0.0
			text	528236	528236	0	0.0
	pigweed-app	nrf52840dk_nrf52840	(read/write)	541835	541835	0	0.0
			bss	52588	52588	0	0.0
			rodata	50104	50104	0	0.0
			text	376940	376940	0	0.0
	pump-app	nrf52840dk_nrf52840	(read/write)	913151	913151	0	0.0
			bss	117940	117940	0	0.0
			rodata	103820	103820	0	0.0
			text	613928	613928	0	0.0
	pump-controller-app	nrf52840dk_nrf52840	(read/write)	908231	908231	0	0.0
			bss	117964	117964	0	0.0
			rodata	102932	102932	0	0.0
			text	609856	609856	0	0.0
	shell	nrf52840dk_nrf52840	(read/write)	798479	798479	0	0.0
			bss	109776	109776	0	0.0
			rodata	78324	78324	0	0.0
			text	533872	533872	0	0.0
		nrf5340dk_nrf5340_cpuapp	(read/write)	711278	711278	0	0.0
			bss	107664	107664	0	0.0
			rodata	72624	72624	0	0.0
			text	451548	451548	0	0.0
p6	all-clusters-app	default	(read/write)	2406600	2406600	0	0.0
			.bss	117764	117764	0	0.0
			.data	2576	2576	0	0.0
			.text	1364864	1364864	0	0.0
	light-app	default	(read/write)	2327832	2327832	0	0.0
			.bss	105520	105520	0	0.0
			.data	2408	2408	0	0.0
			.text	1286096	1286096	0	0.0
	lock-app	default	(read/write)	2296840	2296840	0	0.0
			.bss	104368	104368	0	0.0
			.data	2352	2352	0	0.0
			.text	1255104	1255104	0	0.0
qpg	lighting-app	qpg6105+debug	(read only)	565376	565376	0	0.0
			(read/write)	146936	146936	0	0.0
			.bss	89672	89672	0	0.0
			.data	1060	1060	0	0.0
			.text	560056	560056	0	0.0
	lock-app	qpg6105+debug	(read only)	513868	513868	0	0.0
			(read/write)	146940	146940	0	0.0
			.bss	88240	88240	0	0.0
			.data	984	984	0	0.0
			.text	508548	508548	0	0.0
	persistent-storage-app	qpg6105+debug	(read only)	106848	106848	0	0.0
			(read/write)	146940	146940	0	0.0
			.bss	38512	38512	0	0.0
			.data	288	288	0	0.0
			.text	101528	101528	0	0.0
telink	lighting-app	tlsr9518adk80d	(read/write)	839398	839398	0	0.0
			bss	87468	87468	0	0.0
			noinit	37160	37160	0	0.0
			text	586504	586504	0	0.0