[css-fonts] [palettes] Remove the none value from font-palette

There are a few reasons to remove it:

It's unimplementable on Core Text.
It was added without WG input.
Clearly the font creator wants their clients to use the font's color facilities; otherwise they would not have created them.
One of the motivations for adding it is false: "Both SVG and COLR fonts must have a regular, uncolored glyph outline available." This is untrue - the fallback might have 0 contours. Many (perhaps even "most"!) of the color fonts we've seen have no outline fallback.
Another motivation was misplaced: "Many glyphs in the "emoji" range will only exist in a font with color capabilities" We already have two mechanisms for selecting non-color emoji: both variation selectors and https://drafts.csswg.org/css-fonts/#font-variant-emoji-prop. Text-style emoji is handled with font fallback, not displaying otherwise-unobservable contours inside fonts.
If the page author wants a non-color rendering, they can use a non-color font.

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