Drop: .opacity() mixin because we no longer support IE8

less/button-groups.less

@@ -239,7 +239,7 @@
     input[type="checkbox"] {
       position: absolute;
       z-index: -1;
-      .opacity(0);
+      opacity: 0;
     }
   }
 }

less/carousel.less

@@ -72,7 +72,7 @@
   left: 0;
   bottom: 0;
   width: @carousel-control-width;
-  .opacity(@carousel-control-opacity);
+  opacity: @carousel-control-opacity;
   font-size: @carousel-control-font-size;
   color: @carousel-control-color;
   text-align: center;
@@ -96,7 +96,7 @@
     outline: 0;
     color: @carousel-control-color;
     text-decoration: none;
-    .opacity(.9);
+    opacity: .9;
   }
 
   // Toggles

less/component-animations.less

@@ -2,11 +2,6 @@
 // Component animations
 // --------------------------------------------------
 
-// Heads up!
-//
-// We don't use the `.opacity()` mixin here since it causes a bug with text
-// fields in IE7-8. Source: https://github.com/twbs/bootstrap/pull/3552.
-
 .fade {
   opacity: 0;
   transition: opacity .15s linear;

less/mixins.less

@@ -12,7 +12,6 @@
 
 // Utilities
 @import "mixins/hide-text.less";
-@import "mixins/opacity.less";
 @import "mixins/image.less";
 @import "mixins/labels.less";
 @import "mixins/reset-filter.less";

less/mixins/opacity.less

-// Opacity
-
-.opacity(@opacity) {
-  opacity: @opacity;
-}