nonmoving: Don't join to mark_thread on shutdown

The mark thread is not joinable as we detach from it on creation.

rts/sm/NonMoving.c

@@ -726,7 +726,6 @@ void nonmovingStop(void)
                    "waiting for nonmoving collector thread to terminate");
         ACQUIRE_LOCK(&concurrent_coll_finished_lock);
         waitCondition(&concurrent_coll_finished, &concurrent_coll_finished_lock);
-        joinOSThread(mark_thread);
     }
 #endif
 }