Remove unused import in overview page

Author: tuan-nguen

extensions/ql-vscode/src/stories/Overview.mdx

@@ -1,7 +1,5 @@
 import { Canvas, Meta, Story } from '@storybook/blocks';
 
-import { VSCodeButton } from '@vscode/webview-ui-toolkit/react';
-
 import iframeImage from './images/update-css-variables-iframe.png';
 import stylesImage from './images/update-css-variables-styles.png';
 import bodyImage from './images/update-css-variables-body.png';