Remove unsafe from inherent define_class! impls #3391

Job	Run time
Check MSRV	1m 0s
Check formatting	13s
Lint macOS x86_64	3m 56s
Lint iOS ARMv7s	4m 8s
Lint visionOS Aarch64 simulator	4m 1s
Lint GNUStep + exceptions	1m 32s
Lint header-translator	33s
Lint test-assembly	25s
Test macOS 15 Aarch64	2m 35s
Verify header translator output	2m 29s
Assembly tests	50s
${{ ((matrix.test && 'Test') \|\| 'Build') }} ${{ matrix.name }}	0s
Check framework features	0s
Test Compiler-RT	40s
Test GNUStep with libobjc2 v1.9	1m 30s
Compiler UI	42s
Test GNUStep with libobjc2 v2.0	1m 10s
Test on iOS simulator w. dinghy	0s
Test GNUStep with libobjc2 v2.1 on nightly	1m 0s
Test GNUStep 32bit	1m 25s
	28m 9s

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