Use new focus colour for remove area button

It already adds a black border on focus so doesn’t need the thick lower
border.

app/assets/stylesheets/components/area-list.scss

@@ -48,7 +48,7 @@
         outline: none;
 
         &:before {
-          background: $focus-colour;
+          background: $govuk-focus-colour;
           color: $black;
           border-color: $black;
         }