css(hr): remove box-sizing

See necolas/normalize.css#817

Author: marella

modernize.css

@@ -46,12 +46,11 @@ h1 {
    ========================================================================== */
 
 /**
- * 1. Add the correct box sizing in Firefox.
+ * 1. Add the correct height in Firefox.
  * 2. Show the overflow in Edge and IE.
  */
 
 hr {
-  box-sizing: content-box; /* 1 */
   height: 0; /* 1 */
   overflow: visible; /* 2 */
 }