Remove the shared channel announcement in queries again.

I never found a string I liked, I haven't found a satisfactory
way to deal with the race condition during auto-join, and there
were some complaints about the additional highlight vector (also
potentially interacting badly with the actual query messages).

Looks like a better idea might be to show the current shared
channels in the "topic" area of query tabs. This is subtly diff-
erent from having a timestamped record of the shared channels at
query open time, but at least gives the area more to do and avoids
these problems.

Additionally, removing this again fixes a regression that caused
query tabs not to light up with the message highlight when newly
opened (probably because the command message event overwrote the
highlight event before the event loop cycled enough for priority
sorting to be possible, haven't really worked through it).