Remove separate print stylesheet

[36degrees]

The stylesheets in GOV.UK Frontend are designed to be used with all media, and use print media queries.

Rather than having two stylesheets for screen and print media, update the stylesheets to use a single stylesheet and any custom tech docs styles to use the media queries.

I'm not sure how we'd roll this change out, as the separate print and screen stylesheets appear to be in the template rather than the gem, so this change won't be applied to existing documentation repos.