Remove all remaining support for LK_DRAIN lockmgr locks. LK_DRAIN was a

Remove all remaining support for LK_DRAIN lockmgr locks.  LK_DRAIN was a
mechanism that allowed lock requests to fail while a thread is blocked
disposing of a structure that has not been entirely unlinked from the
rest of the system, but LK_DRAIN's design only works on UP systems or
while the BGL is held.  It cannot be used in concurrent environments.
Just get rid of it entirely.