Remove handling of old 'editsection' class

It's been replaced with 'mw-editsection' and all cached renders on
Wikimedia wikis have expired by now. All backwards-compatibility
occurences have been removed from MediaWiki core already.

Change-Id: Id3731222445b588de8aec0e86892879ac4472636
Merge "Remove handling of old 'editsection' class"