Deprecate $wgDeferredUpdateList

Callers should instead use DeferredUpdates::addUpdate. The
function is superior because it is able to enforce type-hinting
rather than throwing a fatal error. Also it's not a global :)

The only extension still using the global is FlaggedRevs,
for which I've submitted I1a7c6540b2.

Change-Id: Ic59c90c0d0131039295bd785280dc70ebde6e40f
Merge "Deprecate $wgDeferredUpdateList"