Remove emberAfStackDown.

[bzbarsky-apple]

This is unused and does nothing.

Fixes #1940

[github-actions]

PR #25296: Size comparison from b7e3cbb to 7fb8030

Decreases (1 build for cc32xx)

platform	target	config	section	b7e3cbb	7fb8030	change	% change
cc32xx	lock	CC3235SF_LAUNCHXL	.debug_abbrev	930133	930077	-56	-0.0
			.debug_aranges	87304	87280	-24	-0.0
			.debug_frame	299872	299816	-56	-0.0
			.debug_info	20257011	20256908	-103	-0.0
			.debug_line	2656183	2656086	-97	-0.0
			.debug_ranges	281792	281768	-24	-0.0
			.debug_str	3023459	3023294	-165	-0.0

Full report (1 build for cc32xx)

platform	target	config	section	b7e3cbb	7fb8030	change	% change
cc32xx	lock	CC3235SF_LAUNCHXL		0	0	0	0.0
			(read only)	642681	642681	0	0.0
			(read/write)	203672	203672	0	0.0
			.ARM.attributes	44	44	0	0.0
			.ARM.exidx	8	8	0	0.0
			.bss	197072	197072	0	0.0
			.comment	194	194	0	0.0
			.data	1480	1480	0	0.0
			.debug_abbrev	930133	930077	-56	-0.0
			.debug_aranges	87304	87280	-24	-0.0
			.debug_frame	299872	299816	-56	-0.0
			.debug_info	20257011	20256908	-103	-0.0
			.debug_line	2656183	2656086	-97	-0.0
			.debug_loc	2797535	2797535	0	0.0
			.debug_ranges	281792	281768	-24	-0.0
			.debug_str	3023459	3023294	-165	-0.0
			.ramVecs	780	780	0	0.0
			.resetVecs	64	64	0	0.0
			.rodata	105769	105769	0	0.0
			.shstrtab	232	232	0	0.0
			.stab	204	204	0	0.0
			.stabstr	441	441	0	0.0
			.stack	2048	2048	0	0.0
			.strtab	377825	377825	0	0.0
			.symtab	256256	256256	0	0.0
			.text	534788	534788	0	0.0