Intent to unship: prefers-color-scheme:no-preference value.

[Emilio Cobos Álvarez]

Following this CSS resolution[1], I intend to remove the no-preference
value for the prefers-color-scheme media query in bug 1643656[2].

For context, this value never matches in any browser (unless the user
overrides a hidden pref), and was relatively recently introduced, so the
idea is that it should be pretty safe to remove.

In any case, I've added a pref just in case we need to back out the
behavior change: layout.css.prefers-color-scheme-no-preference.enabled,
which will default to false.

Blink already sent out a similar already-approved intent email[3], and
WebKit already has a bug on file[4], plus they supported the change in
previous discussions, see the minutes in the spec issue.

Let me know if there's any concern with this.

Thanks,

-- Emilio

[1]: https://github.com/w3c/csswg-drafts/issues/3857#issuecomment-634779976
[2]: https://bugzilla.mozilla.org/show_bug.cgi?id=1643656
[3]:
https://groups.google.com/a/chromium.org/d/msg/blink-dev/ragu5Is6vCw/bsrP_5O6BAAJ
[4]: https://bugs.webkit.org/show_bug.cgi?id=212952