Remove gtk stylesheets

[Kazhnuz]

GTK stylesheet aren't supported by GNOME, and won't be as easy to use with libadwaita (even if still possible via GTK_THEME, it'll break the apps even more than it already does as they rely on the adwaita stylesheet as an API). So I think it makes sense to not show them. If/when a good official or community replacement or themes start existing, they could replace them.

The customization category could be used for the other nice stuff that we can customize in GNOME, like icons, cursor, extensions, etc.