Remove `approval_rules` feature flag and remove obsolete approvals implementation
After testing we should remove the feature flag entirely. This also involves removing the obsolete parts of the old approvals feature.
This includes:
- https://gitlab.com/gitlab-org/gitlab-ee/issues/8883
- Walk through all the TODOs related to #1979 (closed)
- #9430
Some references unfortunately has to stay until end of 12.x. The follow up issue is https://gitlab.com/gitlab-org/gitlab-ee/issues/12934
Edited by Mark Chao