Issue: Unneeded Last Post Join in vB4

In vBulletin 4, the last post info is handled by the default thread data. This is accounted for in most places by only executing the compatibility updates to last post data in 3.x. However, the data is still fetched (and not used) when the article cache needs to be rebuilt for a whole forum (most commonly, changing existing forums to wiki forums).

Fixed for the next build. This join is now only added if vBulletin < 4.