Remove a listening callback on the specified channel.

async def remove_listener(self, channel, callback):
        """Remove a listening callback on the specified channel."""
        if self.is_closed():
            return
        if channel not in self._listeners:
            return
        if callback not in self._listeners[channel]:
            return
        self._listeners[channel].remove(callback)
        if not self._listeners[channel]:
            del self._listeners[channel]
            await self.fetch('UNLISTEN {}'.format(utils._quote_ident(channel)))