Revert "Merge branch 'ce-33929-allow-to-enable-perf-bar-for-a-group' into 'master'"

This reverts commit 67d897e6, reversing changes made to 7fd30aee.

We changed the approach in https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/12362 to use an application setting instead of gitlab.rb. 🎉