Do not store unique fetches in pack-objects cache
This changes the default of the new pack_objects_cache.min_occurrences setting to 1. Experimentation on GitLab.com shows this reduces the cache disk storage footprint by over 50% while barely reducing the cache hit rate.
While we are here, we also remove the temporary environment variable GITALY_PACK_OBJECTS_CACHE_MIN_OCCURRENCES that we used for experimentation on GitLab.com.
Part of gitlab-com/gl-infra/scalability#2222 (closed).
Changelog: other