Check mergeability in merge to ref service

What does this MR do?

This idea came from @oswaldo's this comment https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/9622#note_155132194

This effectively simplifies https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/9622's logic.

Also, we should remove the feature flag merge_to_tmp_merge_ref_path as per https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/9622#note_155125130

What are the relevant issue numbers?

Related: https://gitlab.com/gitlab-org/gitlab-ee/issues/7380

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• [-] Documentation created/updated via this MR
• [-] Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• [-] Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• [-] Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• Security reports checked/validated by reviewer