MPD: Wrongly annonces support for channels

[mxjeff]

This issue is actually related to an issue found in an utility MPD client MPD_sima (also mentioned in #14).

Here is the ouptput of commands MPD command:

printf "commands\nclose\n" | nc localhost 6600 | grep -E '(subscribe|channels)' -
command: channels
command: subscribe
command: unsubscribe

mopidy should not expose channels capabilities if it does not implement it.

Cheers

[kingosticks]

Yeh, despite the wording:

Shows which commands the current user has access to.

It seems nonsensical to advertise anything that isn't actually implemented (yet).

Either remove the @protocol.commands.add decorator for all these non-implemented commands, or set list_command=False.