doc: remove outdated notes on stdio in workers

Workers support `stdio` streams since the initial PR landed. These
lines are editing leftovers from before that and should be removed.

PR-URL: https://github.com/nodejs/node/pull/23054


Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>