Deprecate 'no-preference' as it is no longer supported

[rishabh-ink]

Background

As mentioned in Detecting the desire for light or dark color schemes: the prefers-color-scheme feature spec,

This feature, like the other prefers-* features, previously had a no-preference value to indicate an author not expressing an active preference. However, user agents converged on expressing the "default" behavior as a light preference, and never matching no-preference.
If a future user agent wishes to expose a difference between "no preference" and "really wants a light display", please contact the CSSWG to discuss this.

Since the "no-preference" value is being replaced by "light" as default, this PR removes it from the codebase and fixes #1.