remove flow entries about the group and send a LEAVE message
        if exists.

def _remove_multicast_group(self, datapath, outport, dst):
        """remove flow entries about the group and send a LEAVE message
        if exists."""
        ofproto = datapath.ofproto
        parser = datapath.ofproto_parser
        dpid = datapath.id

        self._send_event(
            EventMulticastGroupStateChanged(
                MG_GROUP_REMOVED, dst, outport, []))
        self._del_flow_entry(datapath, outport, dst)
        for port in self._to_hosts[dpid][dst]['ports']:
            self._del_flow_entry(datapath, port, dst)
        leave = self._to_hosts[dpid][dst]['leave']
        if leave:
            if ofproto.OFP_VERSION == ofproto_v1_0.OFP_VERSION:
                in_port = leave.in_port
            else:
                in_port = leave.match['in_port']
            actions = [parser.OFPActionOutput(outport)]
            self._do_packet_out(
                datapath, leave.data, in_port, actions)