Disable focus outline on navigation link

As we already apply a focus state to the link child element, there is no need for an additional focus outline on the link itself that User Agents apply as default

lib/assets/stylesheets/modules/_toc.scss

@@ -43,6 +43,7 @@
 
         a:focus {
           text-decoration: none;
+          outline: none;
 
           span {
             @include govuk-focused-text;