stream: simplify uint8ArrayToBuffer helper

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

Refs: https://github.com/nodejs/node/pull/29514

PR-URL: https://github.com/nodejs/node/pull/30041
Fixes: https://github.com/nodejs/node/issues/30040


Reviewed-By: Yongsheng Zhang <zyszys98@gmail.com>
Reviewed-By: David Carlier <devnexen@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Jiawen Geng <technicalcute@gmail.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>