posix: mqmeue: do not return NULL after pthread_exit()

pthread_exit() does not return and therefore it does not make
sense to return NULL after it in mq_notify_thread(), and that
would constitute dead code.

Rather than explicitly exiting the thread, simply return
gracefully from the thread function, and allow the pthread to
terminate in the usual way.

Signed-off-by: Christopher Friedt <cfriedt@meta.com>