Remove backfill migration for legacy projects

What does this MR do?

We've found some problems with the background migration: BackfillLegacyProjectRepositories

See https://gitlab.com/gitlab-org/gitlab-ce/issues/56061

The migration was disabled to run on production: https://gitlab.com/gitlab-com/gl-infra/infrastructure/issues/5906

So to overcome it's not being run anywhere, just remove it.

Whenever all problems in BackfillLegacyProjectRepositories are fixed we'll create a new migration (with a different timestamp) to have it run.

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.
• Security reports checked/validated by reviewer