PLA-1145 disabling cache purge for article API

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.
Merge pull request #2802 from Wikia/PLA-1145

PLA-1145 disabling cache purge for article API