Remove extern_category! #3383

Job	Run time
Check formatting	11s
Check MSRV	1m 6s
Lint macOS x86_64	4m 13s
Lint iOS ARMv7s	4m 15s
Lint visionOS Aarch64 simulator	4m 4s
Lint GNUStep + exceptions	2m 2s
Lint header-translator	40s
Lint test-assembly	39s
Test macOS 15 Aarch64	1m 55s
Assembly tests	50s
Verify header translator output	2m 18s
Check framework features	0s
${{ ((matrix.test && 'Test') \|\| 'Build') }} ${{ matrix.name }}	0s
Test Compiler-RT	32s
Test GNUStep with libobjc2 v1.9	1m 3s
Compiler UI	45s
Test GNUStep with libobjc2 v2.0	1m 6s
Test on iOS simulator w. dinghy	0s
Test GNUStep with libobjc2 v2.1 on nightly	1m 4s
Test GNUStep 32bit	1m 6s
	27m 49s

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