Remove node-fetch polyfill in next.js example

[afzalsayed96]

As per webiny/docs.webiny.com#109

[afzalsayed96]

Oh by the way this example does not work if you follow the docs and define your body as Rich Text

It breaks at the following line as post.body would be an object (not a valid react child)

<p style={{ whiteSpace: "pre-wrap" }}>{post.body}</p>

This should probably be addressed in a separate issue/PR