[Silabs][EFR32] Removing MG21 definitions from WIFI

[poyamini]

• Cleaning up the code relating to MG21 from WIFI

After removing the definitions of MG21, able to build the below combinations:
On WIFI

• MG12 + RS9116
• MG12 + WF200
• MG24 + RS9116
• MG24 + WF200

On Thread

• MG12 + RS9116
• MG12 + WF200
• MG24 + RS9116
• MG24 + WF200

[github-actions]

PR #25217: Size comparison from 3951efb to 1712541

Full report (1 build for cc32xx)

platform	target	config	section	3951efb	1712541	change	% change
cc32xx	lock	CC3235SF_LAUNCHXL		0	0	0	0.0
			(read only)	642545	642545	0	0.0
			(read/write)	203664	203664	0	0.0
			.ARM.attributes	44	44	0	0.0
			.ARM.exidx	8	8	0	0.0
			.bss	197064	197064	0	0.0
			.comment	194	194	0	0.0
			.data	1480	1480	0	0.0
			.debug_abbrev	930168	930168	0	0.0
			.debug_aranges	87272	87272	0	0.0
			.debug_frame	299752	299752	0	0.0
			.debug_info	20254059	20254059	0	0.0
			.debug_line	2655143	2655143	0	0.0
			.debug_loc	2795383	2795383	0	0.0
			.debug_ranges	281592	281592	0	0.0
			.debug_str	3017470	3017470	0	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	534604	534604	0	0.0