Remove af-structs.h, now that it's just empty.

[bzbarsky-apple]

No description provided.

[github-actions]

PR #25093: Size comparison from 80ee243 to 61abfcf

Increases (1 build for cc32xx)

platform	target	config	section	80ee243	61abfcf	change	% change
cc32xx	lock	CC3235SF_LAUNCHXL	.debug_line	2657822	2657824	2	0.0

Full report (1 build for cc32xx)

platform	target	config	section	80ee243	61abfcf	change	% change
cc32xx	lock	CC3235SF_LAUNCHXL		0	0	0	0.0
			(read only)	641857	641857	0	0.0
			(read/write)	204124	204124	0	0.0
			.ARM.attributes	44	44	0	0.0
			.ARM.exidx	8	8	0	0.0
			.bss	197528	197528	0	0.0
			.comment	194	194	0	0.0
			.data	1476	1476	0	0.0
			.debug_abbrev	931515	931515	0	0.0
			.debug_aranges	87432	87432	0	0.0
			.debug_frame	300120	300120	0	0.0
			.debug_info	20338088	20338088	0	0.0
			.debug_line	2657822	2657824	2	0.0
			.debug_loc	2790649	2790649	0	0.0
			.debug_ranges	281568	281568	0	0.0
			.debug_str	3003658	3003658	0	0.0
			.ramVecs	780	780	0	0.0
			.resetVecs	64	64	0	0.0
			.rodata	105697	105697	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	376514	376514	0	0.0
			.symtab	256016	256016	0	0.0
			.text	534036	534036	0	0.0