fix(app): don't send tag_category when field type is not tag_picker

The backend validates tag_category as 'prohibited' for non-tag_picker
field types. Sending null triggered a 422 validation error.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

apps/app/src/components/organisation/RegistrationFieldTemplatesTab.vue

@@ -140,9 +140,6 @@ function onSubmit() {
     if (showTagCategory.value) {
       payload.tag_category = form.value.tag_category || null
     }
-    else {
-      payload.tag_category = null
-    }
 
     if (editingTemplate.value) {
       updateTemplate(