Unsubscribes a socket from a channel.

def unsubscribe(self, socket_id, channel):
        """Unsubscribes a socket from a channel."""
        con = self._get_connection(socket_id, create=False)
        if con is not None:
            con.subscriptions.discard(channel)
        try:
            self.subscriptions[channel].discard(socket_id)
        except KeyError:
            pass