Kernel.Pthreads: Remove unreachable in posix_pthread_mutex_timedlock (#4005)

* Fix assert

Just a typical day of me pushing something a month ago, nobody testing/reviewing it, then finding out it's broken when that code inevitably makes it into production.

* Remove unreachable in posix_pthread_mutex_timedlock

It's apparently something that was added during pthreads rewrite, but the actual code for this function seems to be fully implemented?