Fix revision table cleanup on delete

The ContentHandler branch inappropriately moved this essential
query to content-type-dependent code, despite the fact that revision
table cleanup needs to be done independently of content type. Before
r15707, the revision query was done immediately before the page query,
but it was moved down to allow it to be brought into the same
if (!$dbw->cascadingDeletes() ) block as the link updates. I have moved
it to immediately after the page delete, so that the rollback which has
since been added will not affect the expensive revision table query.

This should fix the database corruption we are seeing on some page
deletions (13 affected pages on enwiki, 14 on nlwiki, etc.)

Bug: 53687
Change-Id: I7a54c4416be07fb8cc7a5698e77927ccb24cbd6e
(cherry picked from commit d3364da9cea7f8bb0b41611f44d185c51f6dd73c)