Deletes an invoice by unique_id. This removes the invoice from the collection and should only be used when the invoice is no longer needed. A partner token may delete its direct-child merchants' invoices, the same as it may create/update them - this is not merchant-only, and no 403 is returned for a partner caller. Paid invoices cannot be deleted (422). This is a soft delete (sets deleted_at/deactivated_at).
