def flow_removed_handler(self, evt):
        """FlowRemoved event handler. when the removed flow entry was
        for LACP, set the status of the slave i/f to disabled, and
        send a event."""
        msg = evt.msg
        datapath = msg.datapath
        ofproto = datapath.ofproto
        dpid = datapath.id
        match = msg.match
        if ofproto.OFP_VERSION == ofproto_v1_0.OFP_VERSION:
            port = match.in_port
            dl_type = match.dl_type
        else:
            port = match['in_port']
            dl_type = match['eth_type']
        if ether.ETH_TYPE_SLOW != dl_type:
            return
        self.logger.info(
            "SW=%s PORT=%d LACP exchange timeout has occurred.",
            dpid_to_str(dpid), port)
        self._set_slave_enabled(dpid, port, False)
        self._set_slave_timeout(dpid, port, 0)
        self.send_event_to_observers(
            EventSlaveStateChanged(datapath, port, False))
FlowRemoved event handler. when the removed flow entry was
        for LACP, set the status of the slave i/f to disabled, and
        send a event.