Undoes attaching a purchase order to a trace: the purchase order (any company's) is unlinked from the sales order at the end of tracingLinkId — a link the caller created — its own link reconnected and the attaching link disconnected, in one transaction. Answers the trace graph of initialOrderId, one of the caller's orders. 404 not_found when the purchase or sales order is missing; 400 unable_to_unlink when the link is missing or not the caller's, the purchase order has no link of its own, the initial order is not the caller's, or no sales order sits at the link's end.