manager: don't display internal placeholders (#19341)

combo/manager/views.py

@@ -211,6 +211,8 @@ class PageView(DetailView):
         combo_template = settings.COMBO_PUBLIC_TEMPLATES.get(template)
 
         for placeholder in self.object.get_placeholders():
+            if placeholder.internal:
+                continue
             placeholder_dict = {
                 'key': placeholder.key,
                 'name': placeholder.name,