Migration cleanup for merge request's commit_count (rev 1)

This MR is cleanup after adding merge request's commit_count column. This is basically revived !17513 (merged) which we had to revert because it turned out that there were still many not-migrated records on gitlab.com (approx 100k - https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/17567#note_61904891). We rescheduled migration in 10.7 which migrated these remaining records - https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/17567#note_68985336.

So now we can run the final cleanup (originally included in !17513 (merged)), which does:

• processes any pending records which are not migrated yet
• bumps import_export version because of new commits_count attribute
• removes commits_count fallback method

Related to:

• #41698 (closed)