Remove use of mixin to hide outline on keyboard focus

The implementation is missing. It will be added in a follow-up PR.

src/styles/sidebar/components/menu-item.scss

@@ -117,6 +117,4 @@ $menu-item-padding: 10px;
     width: 12px;
     height: 12px;
   }
-
-  @include outline-on-keyboard-focus;
 }