stream: simplify isUint8Array helper

The fallback code is no longer used when exporting to readable-stream.

Refs: https://github.com/nodejs/node/pull/29475

PR-URL: https://github.com/nodejs/node/pull/29514


Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: David Carlier <devnexen@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Jiawen Geng <technicalcute@gmail.com>