Remove -webkit-backface-visibility on skew mixin

Ref #5319 - This rule is causing pixelated edges on iOS. Removing
it makes everything smooth across all OS's, browsers, and devices.

less/mixins.less

@@ -212,7 +212,6 @@
       -ms-transform: skewX(@x) skewY(@y); // See https://github.com/twitter/bootstrap/issues/4885
        -o-transform: skew(@x, @y);
           transform: skew(@x, @y);
-  -webkit-backface-visibility: hidden; // See https://github.com/twitter/bootstrap/issues/5319
 }
 .translate3d(@x, @y, @z) {
   -webkit-transform: translate3d(@x, @y, @z);