_s: Remove remaining box-sizing prefix.

Also removes the box-sizing mixin, as it is of no use anymore.

See http://caniuse.com/#feat=css3-boxsizing
See #558.

Fixes #561.

style.css

@@ -77,7 +77,7 @@ html {
 *,
 *:before,
 *:after { /* apply a natural box layout model to all elements; see http://www.paulirish.com/2012/box-sizing-border-box-ftw/ */
-	box-sizing:         border-box;
+	box-sizing: border-box;
 }
 
 body {
@@ -355,8 +355,7 @@ input[type="radio"] {
 
 input[type="search"] {
 	-webkit-appearance: textfield; /* Addresses appearance set to searchfield in S5, Chrome */
-	-webkit-box-sizing: content-box; /* Addresses box sizing set to border-box in S5, Chrome (include -moz to future-proof) */
-	box-sizing:         content-box;
+	box-sizing: content-box;
 }
 
 input[type="search"]::-webkit-search-decoration { /* Corrects inner padding displayed oddly in S5, Chrome on OSX */