muc/subject: fix obsolete FIXME about discoing MUCs
muc/subject: fix obsolete FIXME about discoing MUCs

fd.o#21152 was fixed in 2009. We do have to make the test echo back our
unavailable presence if we want to re-use the JID, but that's fine, we
can do that.

Ziemin/telepathy-gabble,mlundblad/telepathy-gabble,mlundblad/telepathy-gabble,Ziemin/telepathy-gabble,mlundblad/telepathy-gabble,Ziemin/telepathy-gabble,Ziemin/telepathy-gabble