Revert: Vendor whatwg-fetch@3.6.20 (#56414)

Summary:
Pull Request resolved: #56414

Reverting due to some internal applications relying on bugs in older whatwg-fetch versions.

Changelog: [Internal]

Differential Revision: D100389649

fbshipit-source-id: f87ed5e212508af8032e452cf2ba9836292a49d1

Author: bherila

packages/react-native/Libraries/Network/fetch.js

@@ -12,7 +12,7 @@
 
 // side-effectful require() to put fetch,
 // Headers, Request, Response in global scope
-require('./whatwg-fetch');
+require('whatwg-fetch');
 
 export const fetch = global.fetch;
 export const Headers = global.Headers;