Fix migration for removing MR diff indexes

What does this MR do?

This MR fixes the migration that removes the unique index/constraint from merge_request_diffs.merge_request_id

Does this MR meet the acceptance criteria?

• Tests
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if you do - rebase it please)
• Squashed related commits together

cc @stanhu