stream: remove dead code

`String.prototype.split()` returns an array of strings so the branch is
never taken.

Fixes: https://github.com/nodejs/node/issues/30040

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


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>