Cleanup and remove PopulateUntrackedUploads background migration

What does this MR do?

This removes an old background migration which isn't needed anymore (not called from anywhere after migrations have been trimmed down to >= 2019 only).

The background migration seems to have used a temporary table, which might have been left for some installations (e.g. GitLab.com). We also include a migration here to drop said table if it exists.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content