Remove default font styles from cookie banner Sass

Removes the default font styles (setting the font and font-size) from
the cookie banner Sass.

This work was prompted by alphagov/govuk-design-system#1886

With the default font styles present, it's very difficult to spot if you forget
to add the relevant `govuk-body` styles to any `html` you pass into the component.
We had missed this ourselves when we added the cookie banner to the design system.

This change should only affect users who are passing `html` to the cookie banner.

src/govuk/components/cookie-banner/_index.scss

@@ -4,8 +4,6 @@
   $border-bottom-width: govuk-spacing(2);
 
   .govuk-cookie-banner {
-    @include govuk-font($size: 19);
-
     padding-top: govuk-spacing(4);
     // The component does not set bottom spacing.
     // The bottom spacing should be created by the items inside the component.