Approval UI showing up for free plan

Since 11.0, approval form is displayed in the merge request create form even for free plan users. The change was introduced in https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/5635

The form displays depending on return value of requires_approve?. Its implementation was changed:

   def requires_approve?
-    approvals_required.nonzero?
+    # keep until UI changes implemented
+    true
   end

approvals_required being greater than 1 implied that the feature is enabled, so after we changed it to true, we lost the feature implication. This is probably unintentional. and we should add a feature check so we hide it in free plan.