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/_close.scss

@@ -8,13 +8,14 @@
   opacity: .5;
 
   // Override <a>'s hover style
-  @include hover {
+  &:hover {
     color: $close-color;
     text-decoration: none;
   }
 
   &:not(:disabled):not(.disabled) {
-    @include hover-focus {
+    &:hover,
+    &:focus {
       opacity: .75;
     }
   }