Revert "Fix: Contentful webhook body parse." (#40925)

Reverts #40732

x-ref:
#40732 (comment)

examples/cms-contentful/pages/api/revalidate.js

@@ -14,7 +14,9 @@ export default async function handler(req, res) {
   }
 
   try {
-    let postSlug = JSON.parse(req.body).fields.slug['en-US']
+    // Note: if this fails to parse you may have forget to set the
+    // "content-type" header correctly as mentioned here https://github.com/vercel/next.js/blob/canary/examples/cms-contentful/README.md#step-9-try-using-on-demand-revalidation
+    let postSlug = req.body.fields.slug['en-US']
 
     // revalidate the individual post and the home page
     await res.revalidate(`/posts/${postSlug}`)