Remove AsMut and BorrowMut implementations #2994

Job	Run time
Check formatting	7s
Check MSRV	57s
Lint macOS 64bit	2m 29s
Lint iOS 32bit	2m 3s
Lint GNUStep + exceptions	1m 27s
Lint header-translator	27s
Lint test-assembly	24s
Test macOS 12	2m 55s
Verify header translator output	47s
Assembly tests	47s
Check framework features	0s
${{ matrix.name }}	0s
Test Compiler-RT	28s
Test GNUStep with libobjc2 v1.9	59s
Test on iOS simulator w. dinghy	0s
Compiler UI	1m 9s
Test GNUStep with libobjc2 v2.0	1m 6s
Test GNUStep with libobjc2 v2.1 on nightly	51s
Test GNUStep 32bit	1m 12s
	18m 8s

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