message: Drop unused `_mirrored_container` property of variables that are
  components of a MirroredVariable. We switched to using
  `_distributed_container` set in the parent class
  `DistributedVariable`, but the code setting `_mirrored_container` was
  accidentally added back as a result of a merge.
diff: @@ -340,10 +340,6 @@ class MirroredVariable(DistributedVariable, Mirrored,
  """Holds a map from device to variables whose values are kept in sync."""
  def __init__(self, index, primary_var, aggregation):
  - # Use a weakref to make it easy to map from the contained values
  - # to the container without introducing a reference cycle.
  - for v in six.itervalues(index):
  - v._mirrored_container = weakref.ref(self) # pylint: disable=protected-access
  self._primary_var = primary_var
  self._aggregation = aggregation
  super(MirroredVariable, self).__init__(index)
  