Clean up `MigrateLegacyArtifacts` background migrations with `BackgroundMigration.steal`

Description

We scheduled background migrations for migrating
legacy artifacts (e.g. ci_builds.artifacts_file) to the new artifact structure (e.g. ci_job_artifacts.file) in %11.3. We need to clean up the remaining background migration queues in %11.4.

TODO

• Remove the temporary index (partial_index_ci_builds_on_id_with_legacy_artifacts)

Reference

• https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/development/background_migrations.md

/cc @grzesiek @abrandl @ayufan