_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.

sass/forms/_fields.scss

@@ -5,9 +5,7 @@ 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) */
-	-moz-box-sizing:    content-box;
-	box-sizing:         content-box;
+	box-sizing: content-box;
 }
 
 input[type="search"]::-webkit-search-decoration { /* Corrects inner padding displayed oddly in S5, Chrome on OSX */