fix: remove outdated mixin

Author: guifry

src/frontend/scss/main.scss

@@ -312,17 +312,6 @@ h2:focus-visible {
     color: $govuk-inverse-button-text-colour;
   }
 
-  // alphagov/govuk_template includes a specific a:link:focus selector
-  // designed to make unvisited links a slightly darker blue when focussed, so
-  // we need to override the text colour for that combination of selectors so
-  // so that unvisited links styled as buttons do not end up with dark blue
-  // text when focussed.
-  @include _govuk-compatibility(govuk_template) {
-    &:link:focus {
-      color: $govuk-inverse-button-text-colour;
-    }
-  }
-
   &:hover {
     background-color: $govuk-inverse-button-hover-colour;