This removes the deprecated `output.html.copy-fonts` option. This was
deprecated in https://github.com/rust-lang/mdBook/pull/1987. The
behavior now is that the default fonts are copied over unless there is a
custom `theme/fonts/fonts.css` file.
This updates everything for the move of utils to mdbook-core. There will
be followup commits that will be moving and refactoring these utils.
This simply moves them over unchanged (except visibility).
