Stop trying to remove persisted subscriptions when we can't have any.

[bzbarsky-apple]

Specifically, for non-subscription ReadHandlers and for cases when we fail to even set up a subscription id.

[github-actions]

PR #26039: Size comparison from fb13051 to 9d01c96

Increases (1 build for cc32xx)

platform	target	config	section	fb13051	9d01c96	change	% change
cc32xx	lock	CC3235SF_LAUNCHXL	(read only)	642945	642953	8	0.0
			.debug_info	20303341	20303346	5	0.0
			.debug_line	2679963	2679969	6	0.0
			.text	536472	536480	8	0.0

Full report (1 build for cc32xx)

platform	target	config	section	fb13051	9d01c96	change	% change
cc32xx	lock	CC3235SF_LAUNCHXL		0	0	0	0.0
			(read only)	642945	642953	8	0.0
			(read/write)	203848	203848	0	0.0
			.ARM.attributes	44	44	0	0.0
			.ARM.exidx	8	8	0	0.0
			.bss	197248	197248	0	0.0
			.comment	194	194	0	0.0
			.data	1480	1480	0	0.0
			.debug_abbrev	933129	933129	0	0.0
			.debug_aranges	87616	87616	0	0.0
			.debug_frame	301352	301352	0	0.0
			.debug_info	20303341	20303346	5	0.0
			.debug_line	2679963	2679969	6	0.0
			.debug_loc	2824782	2824782	0	0.0
			.debug_ranges	286208	286208	0	0.0
			.debug_str	3039557	3039557	0	0.0
			.ramVecs	780	780	0	0.0
			.resetVecs	64	64	0	0.0
			.rodata	104353	104353	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	377533	377533	0	0.0
			.symtab	256800	256800	0	0.0
			.text	536472	536480	8	0.0