Reverted "FOR UPDATE" in 61f246aa786d824d0653522ed679c16be719da80.
- Previously, this code would select FOR UPDATE unless ALF_NO_LINK_LOCK
was set in $wgAntiLockFlags (which in fact was set for wmf sites for ages
to reduce contention). In this case, it was just loading from the master,
without locking, and relying on the page_latest check. Now, it just never
selects FOR UPDATE, regardless of the lock flags, relying on page_latest.