Remove dead code from umtx support:
- Retire long time unused (basically always unused) sys__umtx_lock()
  and sys__umtx_unlock() syscalls
- struct umtx and their supporting definitions
- UMUTEX_ERROR_CHECK flag
- Retire UMTX_OP_LOCK/UMTX_OP_UNLOCK from _umtx_op() syscall

__FreeBSD_version is not bumped yet because it is expected that further
breakages to the umtx interface will follow up in the next days.
However there will be a final bump when necessary.

Sponsored by:	EMC / Isilon storage division
Reviewed by:	jhb
Regen per r263318.

Sponsored by:	EMC / Isilon storage division