Pathway to D17685. This column is (mostly) a denormalization of dateModified on the thread.
Just use a JOIN instead.
This isn't exactly the same: we'll bump threads to the top now for non-message changes (e.g., a topic or title change). That seems fine, but we could put a lastMessageDate on Thread later if we want to refine it.
Also got rid of a lot of other unused stuff. There's a big garbage TODO here, I'll fix that in the next change.