Fix linter warning

Author: fzaninotto

packages/ra-ui-materialui/src/layout/Notification.spec.tsx

@@ -1,5 +1,5 @@
 import * as React from 'react';
-import { render, screen, waitFor } from '@testing-library/react';
+import { render, screen } from '@testing-library/react';
 
 import { ConsecutiveUndoable } from './Notification.stories';