I see know that you must mean that the backups never completed - sorry about that. I would think that they are safe to delete but I do think that they should go away when the backup completes. However, if the backup is not completing you are probably hitting a timeout somewhere...you might check your php settings.
I guess I maybe misread. You said they were unfinished courses' backups which I took to mean that you had courses in the process of restoring that were being backed up.