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/mixins/_list-group.scss

@@ -6,7 +6,8 @@
     background-color: $background;
 
     &.list-group-item-action {
-      @include hover-focus {
+      &:hover,
+      &:focus {
         color: $color;
         background-color: darken($background, 5%);
       }