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>

PORT-FROM: joyent/node @ 0c50195071a57bc40df82c8f57d341a435ff1eb6
PR-URL: nodejs/node#1770
Reviewed-By: Roman Reiss <me@silverwind.io>
Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>