Fix `debusine-admin delete_expired`: cannot delete work requests because of foreign keys
Found this error:
CommandError: Database error: ("Cannot delete some instances of model 'WorkRequest' because they are referenced through protected foreign keys: 'CollectionItem.created_by_workflow', 'CollectionItem.removed_by_workflow'.", ...)
happens in debusine.debian.net nightly.
Edited by Carles Pina i Estany