CLEANUP: thread: remove thread_sync_release() and thread_sync_mask

CLEANUP: thread: remove thread_sync_release() and thread_sync_mask

This function was added in 2.0 when reworking the thread isolation
mechanism to make it more reliable. However it if fundamentally
incompatible with the full isolation mechanism provided by
thread_isolate_full() since that one will wait for all threads to
become idle while the former will wait for all threads to finish
waiting, causing a deadlock.

Given that it's not used, let's just drop it entirely before it gets
used by accident.