Remove `approval_rules` feature flag and remove obsolete approvals implementation

There is a CE port https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/28979

What does this MR do?

The new approval rule system is stable now, we want to remove the legacy approver implementation code.

Functionality-wise the new system is a superset of the old implementation, so it is safe to remove.

Remove approval_rules feature flag and remove obsolete approvals implementation

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created
• [x Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Performance and testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers

Closes #10685 (closed)