Revert "Cleanup after adding MR diff's commit_count"

What does this MR do?

It reverts cleanup code for commits_count column. This cleanup was merged in !17513 (merged) but after merging it it turned out there may be still records with unset commits_count column. So the plan is to re-schedule background migration which sets commits_count, and add cleanup code in the next cycle: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/ #note_62131221