CSS: Avoid (in)visible mixins

public/css/icinga/mixins.less

@@ -41,11 +41,6 @@
   }
 }
 
-.invisible {
-  // Maintain layout but hide visually and from screen readers
-  visibility: hidden;
-}
-
 .opacity(@opacity: 0.6) {
   -webkit-opacity: @opacity;
      -moz-opacity: @opacity;
@@ -77,10 +72,6 @@
           transition: @transition;
 }
 
-.visible {
-  visibility: visible;
-}
-
 // Fadein animation
 
 /* Chrome, WebKit */

public/css/icinga/spinner.less

@@ -20,10 +20,10 @@ div.spinner {
   vertical-align: middle;
 
   i {
-    .invisible();
+    visibility: hidden;
 
     &.active {
-      .visible();
+      visibility: visible;
 
       &:before {
         .animate(spin 2s infinite linear);