UI: Clear undo stack in ClearSceneData() instead

Makes more sense to clear it here.

UI/window-basic-main-scene-collections.cpp

@@ -554,8 +554,6 @@ void OBSBasic::ChangeSceneCollection()
 
 	UpdateTitleBar();
 
-	undo_s.clear();
-
 	if (api)
 		api->on_event(OBS_FRONTEND_EVENT_SCENE_COLLECTION_CHANGED);
 }

UI/window-basic-main.cpp

@@ -4386,6 +4386,8 @@ void OBSBasic::ClearSceneData()
 	if (api)
 		api->on_event(OBS_FRONTEND_EVENT_SCENE_COLLECTION_CLEANUP);
 
+	undo_s.clear();
+
 	disableSaving--;
 
 	blog(LOG_INFO, "All scene data cleared");