Font Library: remove font family and font face preview keys from theme.json schema

[matiasbenedetto]

What?

Remove font family and font face preview keys from theme.json schema.

Why?

During the implementation work of the Font Library, in #56793, we added a preview key to theme.json to persist a preview image of the font family. The preview image had the objective of speeding up the font family preview display by reducing network usage by loading just a tiny image instead of a complete font asset.

Later in the implementation process, we figured out that this won't be strictly necessary for the fonts present in theme.json/global styles due to they probably will be already loaded in the browser to use in the editor. Because of this, I think It would be fine to revert this change that doesn't seem crucial for the performance of the Font Library and could add an unnecessary change for theme.json.

[creativecoder]

Keeping the font preview file out of theme.json (and instead having it as part of Font Library metadata) makes sense to me.

I think the font definitions in theme.json should generally be about rendering the fonts for use as typography settings, and not storing additional metadata we want have for the Font Library.