the process when the querier received a LEAVE message.

def _do_leave(self, leave, in_port, msg):
        """the process when the querier received a LEAVE message."""
        datapath = msg.datapath
        parser = datapath.ofproto_parser

        self._mcast.setdefault(leave.address, {})
        if in_port in self._mcast[leave.address]:
            self._del_flow_entry(
                datapath, in_port, leave.address)
            del self._mcast[leave.address][in_port]
            actions = []
            for port in self._mcast[leave.address]:
                actions.append(parser.OFPActionOutput(port))
            if len(actions):
                self._set_flow_entry(
                    datapath, actions, self.server_port, leave.address)
            else:
                self._del_flow_entry(
                    datapath, self.server_port, leave.address)