Monorepo Jest - remove now unnecessary Babel registration (#38146)

Summary:
Pull Request resolved: #38146

Now that `metro-react-native-babel-transformer` may be loaded from source without transformation, we can remove this workaround / potential footgun.

Jest uses Babel imperatively and doesn't rely on registration (require hooks), so registering within the preprocessor potentially conflicts and has caused problems before. This just tidies up some complexity we don't need.

Changelog: [Internal]

Reviewed By: christophpurrer

Differential Revision: D47162728

fbshipit-source-id: 06215f9bb7b8e71bcca0429d0bae9f20a22f45a8

Author: robhogan

jest/preprocessor.js

@@ -28,17 +28,6 @@ const nodeFiles = /[\\/]metro(?:-[^/]*)[\\/]/;
 // hook. This is used below to configure babelTransformSync under Jest.
 const {only: _, ...nodeBabelOptions} = metroBabelRegister.config([]);
 
-// Register Babel to allow the transformer itself to be loaded from source.
-if (process.env.FBSOURCE_ENV) {
-  // Internal: Use `@fb-scripts/babel-register` to re-use internal
-  // registration, rather than potentially clobbering it and conflicting with
-  // other Jest projects running in the same process.
-  // This package should *NOT* be a dependency of `@react-native/monorepo`.
-  // $FlowIgnore[cannot-resolve-module]
-  require('@fb-scripts/babel-register');
-} else {
-  metroBabelRegister([nodeFiles]);
-}
 const transformer = require('metro-react-native-babel-transformer');
 
 module.exports = {