Remove some no-longer-used ZCL message buffer processing code.

[bzbarsky-apple]

No description provided.

[github-actions]

PR #25166: Size comparison from 07b9bc7 to 6dc7f53

Decreases (1 build for cc32xx)

platform	target	config	section	07b9bc7	6dc7f53	change	% change
cc32xx	lock	CC3235SF_LAUNCHXL	.debug_abbrev	930533	930212	-321	-0.0
			.debug_aranges	87392	87272	-120	-0.1
			.debug_frame	300100	299752	-348	-0.1
			.debug_info	20256440	20254137	-2303	-0.0
			.debug_line	2656005	2655111	-894	-0.0
			.debug_loc	2796891	2795280	-1611	-0.1
			.debug_ranges	281688	281568	-120	-0.0
			.debug_str	3018892	3017662	-1230	-0.0

Full report (1 build for cc32xx)

platform	target	config	section	07b9bc7	6dc7f53	change	% change
cc32xx	lock	CC3235SF_LAUNCHXL		0	0	0	0.0
			(read only)	642529	642529	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	930533	930212	-321	-0.0
			.debug_aranges	87392	87272	-120	-0.1
			.debug_frame	300100	299752	-348	-0.1
			.debug_info	20256440	20254137	-2303	-0.0
			.debug_line	2656005	2655111	-894	-0.0
			.debug_loc	2796891	2795280	-1611	-0.1
			.debug_ranges	281688	281568	-120	-0.0
			.debug_str	3018892	3017662	-1230	-0.0
			.ramVecs	780	780	0	0.0
			.resetVecs	64	64	0	0.0
			.rodata	105817	105817	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	377598	377598	0	0.0
			.symtab	256144	256144	0	0.0
			.text	534588	534588	0	0.0