db: drop Repository CacheInvalidation The benefit of this functionality is questionable. Especially in bigger setups with multiple front-end instances all serving the same multitude of repositories, making the hit rate very low. And the overhead of storing cache invalidation data *in* the database is non-trivial. We preserve a small cache in Repository SA records, but should probably just in general know what we are doing and not ask for the same information multiple times in each request.