Drop all hover mixins

Previously deprecated in v4.x, this clears out the now unused Sass option and removes some unused mixins. Arguably we could remove more, but I like the hover-focus mixin and we make extensive use of it across the project.

scss/_list-group.scss

@@ -23,7 +23,8 @@
   text-align: inherit; // For `<button>`s (anchors inherit)
 
   // Hover state
-  @include hover-focus {
+  &:hover,
+  &:focus {
     z-index: 1; // Place hover/focus items above their siblings for proper border styling
     color: $list-group-action-hover-color;
     text-decoration: none;