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Simplify subclasses list, remove from Box #758

Job	Run time
ruby-bench (--warmup=1 --bench=1)	14m 52s
make (check, cppflags=-DVM_CHECK_MODE)	10m 13s
make (check, --disable-yjit)	7m 49s
make (test-bundled-gems)	20m 32s
make (check, --enable-shared --enable-load-relative)	7m 18s
make-ibm	0s
make (check, i686)	11m 26s
make (check, ubuntu-24.04, capi)	9m 43s
make (test-bundler-parallel, 50)	23m 52s
Ubuntu result	3s
	1h 45m 48s