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 <[email protected]>
Signed-off-by: Julien Gilli <[email protected]>

PORT-FROM: joyent/node @ 0c50195
PR-URL: #1770
Reviewed-By: Roman Reiss <[email protected]>
Reviewed-By: Jeremiah Senkpiel <[email protected]>