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.