Remove quarantine_push_size_check feature flag

In https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/13460, we implemented the functionality to check if push exceeds the repository size limit using the quarantine directory. A feature flag (enabled by default) named quarantine_push_size_check was added so we can easily disable the functionality and revert back to old behavior if problem occurs.

It's been running on production for less than a month now (12.0) as of this writing. We should remove this feature flag in a feature release.