[do-not-merge] Stop using custom fonts on print sheets

Experimenting with switching fonts back to Helvetica, Arial, sans-serif for printing to avoid issues with browser compatibility/OS-specific problems/printer driver broken replacement fonts. 
This is getting overridden by `govuk-template-print.css`, which sets the `nta` stack on `body`.

I'd stick the `font-family` on body here too.

(I'm assuming the non-testing fix for this will be baked into govuk_template?)