Fix deprecated hooks not having a non-deprecated alternative

The implementation in ChangesListSpecialPage::doMainQuery() is
dead code. It's never executed because the relevant subclasses
override the method completely without ever calling
parent::doMainQuery().

This means that the deprecation of the hooks in the subclasses
leaves us with no alternative. Which is a real problem in
Wikibase, see I27716275e966147ee26d81d8ce3f14951937e718.

The same deprecation in ChangesListSpecialPage::getCustomFilters()
works because all suclasses I'm aware of call the parent method.
But please note that extensions may still break if they do this
different. I can not checked them all.

If you want to be sure you should just revert patch
I9cceda5d2dcfc53c852c5682c466b48ad8f31202 that introduced this
non-functional hook. I'm sorry to say that but I wonder how this
passed review.

Change-Id: I7ba3ea64cb145c06011a856e5b56399da4f42339
Merge "Fix deprecated hooks not having a non-deprecated alternative"