Delete a billing plan by reference (TBP-806). Only a plan no workspace is on; one that workspaces are on is refused with 409 `PLAN_IN_USE` and what to do instead (archive it, or move or delete those workspaces first). Requires the `APP_DELETE` privilege on the calling API token. Destructive privileges are opt-in and are never granted by default, so a token without it is refused with 403 (TBP-552).