_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/_reset.scss

@@ -25,7 +25,7 @@ html {
 *,
 *:before,
 *:after { /* apply a natural box layout model to all elements; see http://www.paulirish.com/2012/box-sizing-border-box-ftw/ */
-	@include box-sizing(border-box);
+	box-sizing: border-box;
 }
 body {
 	background: $color__background-body; /* Fallback for when there is no custom background color defined. */

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 */

sass/mixins/_mixins-master.scss

@@ -4,13 +4,6 @@
 	font-size: $sizeValue + rem;
 }
 
-// Box model
-@mixin box-sizing($box-model) {
-	-webkit-box-sizing: $box-model; // Safari <= 7
-	-moz-box-sizing: $box-model; // Firefox <= 19
-	box-sizing: $box-model;
-}
-
 // Border radius
 @mixin border-radius($radius) {
 	background-clip: padding-box; /* stops bg color from leaking outside the border: */

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 */