message: Remove a remove_checkpoint deprecation warning showing up in a 2.x API
  Inlines the only useful bits
diff: @@ -621,7 +621,8 @@ class CheckpointManager(object):
  >= self._last_preserved_timestamp)):
  self._last_preserved_timestamp = timestamp
  continue
  - remove_checkpoint(filename)
  + _delete_file_if_exists(filename + ".index")
  + _delete_file_if_exists(filename + ".data-?????-of-?????")
  def _record_state(self):
  """Saves the `CheckpointManager`'s state in `directory`."""
  