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/_carousel.scss

@@ -101,7 +101,8 @@
   @include transition($carousel-control-transition);
 
   // Hover/focus state
-  @include hover-focus {
+  &:hover,
+  &:focus {
     color: $carousel-control-color;
     text-decoration: none;
     outline: 0;