'Deletes a group_snapshot.
:param context: the context of the caller.
:param group_snapshot: the GroupSnapshot object to be deleted.
:param snapshots: a list of snapshot objects in the group_snapshot.
:returns: model_update, snapshots_model_update'

def delete_group_snapshot(self, context, group_snapshot, snapshots):

    if (not utils.is_group_a_cg_snapshot_type(group_snapshot)):
        raise NotImplementedError()
    cgsnapshot_id = group_snapshot.id
    cg_name = ('cg_snap-' + cgsnapshot_id)
    (model_update, snapshots_model) = self._helpers.delete_consistgrp_snapshots(cg_name, snapshots)
    return (model_update, snapshots_model)
