worker: remove `--experimental-worker` flag

Having an experimental feature behind a flag makes change
if we are expecting significant breaking changes to its API.

Since the Worker API has been essentially stable since
its initial introduction, and no noticeable doubt about
possibly not keeping the feature around has been voiced,
removing the flag and thereby reducing the barrier to experimentation,
and consequently receiving feedback on the implementation,
seems like a good idea.

PR-URL: https://github.com/nodejs/node/pull/25361


Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Yuta Hiroto <hello@hiroppy.me>
Reviewed-By: Shingo Inoue <leko.noor@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Tiancheng "Timothy" Gu <timothygu99@gmail.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: Masashi Hirano <shisama07@gmail.com>
Reviewed-By: Weijia Wang <starkwang@126.com>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>