Revert "Introduce Fiber Scheduler `blocking_region` hook. (#11963)" (… #781

Job	Run time
make (check, cppflags=-DVM_CHECK_MODE)	8m 17s
make (check, i686)	10m 53s
make (check, --disable-yjit)	6m 27s
make (check, --enable-shared --enable-load-relative)	6m 59s
make (check, true, --with-shared-gc=/home/runner/ruby_gc)	8m 13s
make (test-bundler-parallel, 50)	26m 34s
make (test-bundled-gems)	18m 27s
make (check, ubuntu-20.04)	9m 44s
make (check, ubuntu-24.04)	9m 6s
Ubuntu result	1s
	1h 44m 41s

Provide feedback