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/_dropdown.scss

@@ -143,7 +143,8 @@
     }
   }
 
-  @include hover-focus {
+  &:hover,
+  &:focus {
     color: $dropdown-link-hover-color;
     text-decoration: none;
     @include gradient-bg($dropdown-link-hover-bg);