Remove legacy GitlabReferenceCounter / redis code

gitlab-shell can perform pre-receive and post-receive hook actions either through the API or by pushing jobs directly into sidekiq using a redis connection.

When gitlab-shell is running on gitaly nodes, sidekiq-via-redis integration should not be available. We should just remove the fallback methods - they were put in place to ease migration between version (9.3, 9.4, 9.5) rather than resiliency.