Drop unique index on ci_job_artifacts

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/138745



Merged-by: Andy Soiron <asoiron@gitlab.com>
Approved-by: Tianwen Chen <tchen@gitlab.com>
Approved-by: Andy Soiron <asoiron@gitlab.com>
Reviewed-by: Tianwen Chen <tchen@gitlab.com>
Co-authored-by: Maxime Orefice <morefice@gitlab.com>