'Deletes a group.
:param ctxt: the context of the caller.
:param group: the Group object of the group to be deleted.
:param volumes: a list of Volume objects in the group.
:returns: model_update, volumes_model_update'

def delete_group(self, ctxt, group, volumes):

    if volume_utils.is_group_a_cg_snapshot_type(group):
        return self._delete_consistencygroup(ctxt, group, volumes)
    raise NotImplementedError()
