[OpenMP] Fix shutdown hang/race bug

The deadlock/race happens when primary thread gets initz lock and tries to join
the worker thread which waits for the same lock in TLS key destructor.
The patch removes the lock and the code of setting TLS value which needed
the lock. Also removed setting TLS from __kmp_unregister_root_current_thread.

Differential Revision: https://reviews.llvm.org/D90647