Extensions: only hide the add-ons UI when the panel has been extended

Now disabling the extensions add-on restores the original UI.

scripts/startup/bl_ui/space_userpref.py

@@ -2187,7 +2187,7 @@ class USERPREF_PT_addons(AddOnPanel, Panel):
         prefs = context.preferences
 
         use_extension_repos = prefs.experimental.use_extension_repos
-        if use_extension_repos:
+        if use_extension_repos and self.is_extended():
             # Rely on the draw function being appended to by the extensions add-on.
             return