Fix non-reentrant threadpool (#5714)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/5714

If we tried to reenter parallel_for, we would deadlock due to trying to acquire the threadpool mutex, which was already held. Removing the mutex (which is unnecessary) didn't fix the problem because pthreadpool has its own internal mutex.

Reviewed By: kimishpatel

Differential Revision: D62052200

fbshipit-source-id: 1be16d581970fa7b6a84a2f5b4e348b4f5069c80