[PATCH cleanup] vm.c: remove unused USE_THREAD_RECYCLE ifdefs
This is a totally unused ifdef branch, and does not do anything even
when it is enabled, so it only confuses new developers.
Will commit in a few days unless there's objections.
vm.c: remove unused USE_THREAD_RECYCLE [misc #10198]
Unused feature, and it is a no-op even if enabled.