chore: revert playground/mdx/vite.config.ts

Author: hi-ogawa

playground/mdx/vite.config.ts

@@ -6,12 +6,7 @@ import mdx from '@mdx-js/rollup'
 export default defineConfig({
   server: { port: 8901 /* Should be unique */ },
   plugins: [
-    {
-      enforce: 'pre',
-      ...mdx({
-        format: 'mdx',
-      }),
-    },
+    { enforce: 'pre', ...mdx() },
     react({ include: /\.(mdx|md|ts|tsx)$/ }),
   ],
 })