Remove redundant fallback font names and scripts

This site is designed to be viewed in a modern browser with at least 2% global usage. We don't need to include any defensive font fallbacks for older browsers. In particular, there's no need for the long list of fallback font names (which I refer to as "font soup"). The site loads a particular set of custom web fonts (Inter, Urbanist, and Source Code Pro). The stylesheet should refer to only these fonts plus one generic font family as the fallback. Let the browser pick a suitable font based on the generic font family if it cannot support woff2. We also don't need any font loading fallback scripts either. This change will make the site easier to maintain and provide a consistent look across browsers that we support.