Do not part from channels in destroy handler

[daniellandau]

In case the client suddenly disappears (e.g. crashes) the user was
previously parted from the channels. This is terrible behaviour when the
user is using a bouncer, as they expect to stay in their channels. This
handler doesn't seem to be called on clean exit of clients, only when
they disappear suddenly.

This bug: https://bugs.freedesktop.org/show_bug.cgi?id=98685

[gkiagia]

So, the motive for this patch makes total sense, but the actual change is more tricky than it seems.

FYI, the _destroy() method is documented here: https://telepathy.freedesktop.org/spec/Channel_Interface_Destroyable.html#Method:Destroy

As the spec says, this method is meant to be called when the channel should close and all pending messages should be discarded. This is being called by mission-control when the handler crashes, in order to avoid respawning the handler and potentially doing an infinite loop of crashing and respawning... By removing the "part" command, this method practically becomes a no-op; destroy() no longer does anything, so it is incorrect. It should, at least, discard any pending messages.

[daniellandau]

In essence part_from_channel does (in pseude code, the real thing is in idle-muc-channel.c:1284)

	send_command(idleMUCChannel, "PART [channel name]");

so it's not not discarding any pending messages now either.

Even an infinite of crashing and respawning is better than the current behavior, which in essence is irrevocable data loss, as the user gets parted from channels in their bouncer, possibly without them noticing and they might lose a lot of messages.