docs: delete unused/duplicate css files

 - `sh.css` already exists in `api_assets`
 - `sh_vim-dark.css` is unused, but used in the repo `node-website`
        now

Reviewed-by: Trevor Norris <trev.norris@gmail.com>
Signed-off-by: Julien Gilli <julien.gilli@joyent.com>