Handle removed audio group member.

def multizone_member_removed(self, member_uuid):
        """Handle removed audio group member."""
        casts = self._casts
        if member_uuid not in casts:
            casts[member_uuid] = {'listeners': [], 'groups': set()}
        casts[member_uuid]['groups'].discard(self._group_uuid)
        for listener in list(casts[member_uuid]['listeners']):
            listener.removed_from_multizone(self._group_uuid)