Use $govuk-assets-path

We were configuring both $govuk-fonts-path and $govuk-images-path. This
was overly verbose, since for our folder structure we can configure
$govuk-assets-path and it will generate values for $govuk-fonts-path and
$govuk-images-path identical to those we were passing in.

app/frontend/entrypoints/application.scss

@@ -1,5 +1,4 @@
-$govuk-images-path: "@govuk/assets/images/";
-$govuk-fonts-path: "@govuk/assets/fonts/";
+$govuk-assets-path: "@govuk/assets/";
 $govuk-global-styles: true;
 $govuk-suppressed-warnings: (
   ie8