Remove usage of non-existent file

I had an error:

```bash
Error: Cannot find module 'C:\Users\Jerry\projects\jerrygreen.me\node_modules\next-mdx-remote\index.js'. Please verify that the package.json has a valid "main" entry
```

Author: jerrygreen

package.json

@@ -11,7 +11,6 @@
   "bugs": {
     "url": "https://github.com/hashicorp/next-mdx-remote/issues"
   },
-  "main": "index.js",
   "scripts": {
     "lint": "prettier --write \"./**/*.{js,jsx,json}\"",
     "release:major": "release major && npm publish",