Receiving items against a purchase order now makes use of the API forms

- Delete old unused code
- Improve serializer validation

InvenTree/templates/js/translated/forms.js

@@ -1515,6 +1515,7 @@ function initializeChoiceField(field, fields, options) {
     select.select2({
         dropdownAutoWidth: false,
         dropdownParent: $(options.modal),
+        width: '100%',
     });
 }