Remove AtomicReferenceMapBackend and CheapLockable

* Both classes are internal.
* It's quite complicated and not really used anymore,
  except on TruffleRuby <= 21.1 which is very old.
  We can use SynchronizedMapBackend there instead.
* It's the only user of CheapLockable and that looks quite unsafe,
  for example #cheap_wait does not account for spurious wakeups.

Author: eregon