Port to EE: Cleanup legacy artifact background migration

What does this MR do?

EE Version of https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/24144

There was a small conflict in schema.rb

• CE: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/24144
• EE: Here

What are the relevant issue numbers?

Close https://gitlab.com/gitlab-org/gitlab-ce/issues/46866

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• [-] Documentation created/updated
• 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.
• EE specific content should be in the top level /ee folder
• [-] For a paid feature, have we considered GitLab.com plans, how it works for groups, and is there a design for promoting it to users who aren't on the correct plan?
• Security reports checked/validated by reviewer