Remove override to make beta banner blue

Previously the beta bar was orange, which we didn’t want. So we wrote
some SASS to make it blue.

Now that GOV.UK Frontend Toolkit makes it blue, we can remove this SASS.

See alphagov/govuk_frontend_toolkit#370

app/assets/stylesheets/app.scss

@@ -46,8 +46,7 @@
 }
 
 .beta-badge {
-  @include phase-tag(beta);
-  background: $govuk-blue;
+  @include phase-tag();
   margin: 10px 0 0 0;
 }