Signalling message: Remove from group

Removes a channel from a group. The channel name is the server channel name.

This message blocks the client thread until a response is received from the server.

Action	Client -> Server	Server -> Client
Initial action	Message: b"GROUPDIS" Properties: group_name (bytes): Name of group. channel_name (bytes): Name of channel.	Remove channel from group. Respond: -> Complete -> Exception
Complete	Message: b"COMPLETE"	No further action.
Exception	Message: b"EXCEPTON" Properties: exception (str): Exception description.	No further action.