PLA-1145 disabling cache purge for article API (backporting to release)

WikiaDataAccess::cachePurge( self::getCacheKey( $cat, self::CATEGORY_CACHE_ID ) ); is purging wrong keys anyways. If we need better consistency sematntics of getTop and getList endpoints we need to do significant work and we would consider this a feature request.

Backporting #2802 

@ArturD / @prein / @michalroszka