Clean-up caching importer

What

Follow-up from !138357 (merged), we have to clean up the caching importer, and corresponding specs.

Proposal

• Update the lib/gitlab/cache/import/caching.rb with the following:

def self.with_redis(&block)
  Gitlab::Redis::SharedState.with(&block)
end

• Update corresponding specs by removing Gitlab::Redis::SharedState stub part, and replacing clean_gitlab_redis_cache with clean_gitlab_redis_shared_state .

note: it has to in release M+1!