gc: remove comments and change VM_CHECK_MODE #12837

Job	Run time
ruby-bench (--warmup=1 --bench=1 --excludes=shipit)	14m 1s
make (check, --enable-shared --enable-load-relative)	4m 43s
make (check, --disable-yjit)	4m 50s
make (check, cppflags=-DVM_CHECK_MODE)	4m 37s
make (test-bundled-gems)	13m 31s
make (test-bundler-parallel, 50)	21m 58s
make (check, ubuntu-24.04, capi)	6m 2s
make (check, i686)	3m 4s
make-ibm	0s
Ubuntu result	4s
	1h 12m 50s

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