Remove artifacts that came from the datastore.

def _UnregisterDatastoreArtifacts(self):
    """Remove artifacts that came from the datastore."""
    to_remove = []
    for name, artifact in iteritems(self._artifacts):
      if artifact.loaded_from.startswith("datastore"):
        to_remove.append(name)
    for key in to_remove:
      self._artifacts.pop(key)