gitserver: Remove expensive count operation

Created by: eseliger

This count needs to walk the entire table which isn't cheap on Cloud. Since the metric wasn't used in dashboards, I decided to just remove it. If you think this is of value, I suggest that we either use a fuzzy value here, or that we disable it on dotcom only. Also, this metric was incorrect in the current state, because it didn't account for options.OnlyWithoutShard = true and used blocked IS NULL, which IterateGitserverRepos doesn't.

Q: Should we add the blocked IS NULL condition to IterateGitserverRepos as well?