Remove deprecated wikidiff2 configuration
Remove deprecated wikidiff2 configuration

Wikidiff2 integration is done entirely through PHP initialization now, and
the MediaWiki config will throw deprecation errors.

Bug: T210172
Change-Id: I5a949aa2f4647b8e18326513cd79470932d8f728

wikimedia/mediawiki-vagrant,wikimedia/mediawiki-vagrant,wikimedia/mediawiki-vagrant,wikimedia/mediawiki-vagrant,wikimedia/mediawiki-vagrant,wikimedia/mediawiki-vagrant