Remove custom handling for prebundled react env (#49954)
This removes the previous handling for the prebundled react env we were
setting in favor of https://github.com/vercel/vercel/pull/9974
We need to wait to land this until
https://github.com/vercel/vercel/pull/9974 has been rolled
Fixes: https://github.com/vercel/next.js/issues/49169
Fixes: https://github.com/vercel/next.js/issues/49753
x-ref: https://github.com/vercel/next.js/issues/48948
