[UI] Remove placeholder text (#10569)

- Remove TODO entry
- Placeholder when adding external build order support

src/frontend/src/pages/build/BuildDetail.tsx

@@ -238,17 +238,6 @@ export default function BuildDetail() {
         icon: 'manufacturers',
         hidden: !build.external
       },
-      {
-        type: 'text',
-        name: 'purchase_order',
-        label: t`Purchase Order`,
-        icon: 'purchase_orders',
-        copy: true,
-        hidden: !build.external,
-        value_formatter: () => {
-          return 'TODO: external PO';
-        }
-      },
       {
         type: 'text',
         name: 'reference',