CSS: Clean up memory leak in reliableMarginRight

(cherry-picked from 7d15b4d1f181de8868c375c545a51e4dfc28a611)

Fixes gh-1795
Closes gh-1893

Thanks for the report flexphperia!

src/css/support.js

@@ -83,6 +83,7 @@ define([
 				ret = !parseFloat( window.getComputedStyle( marginDiv, null ).marginRight );
 
 				docElem.removeChild( container );
+				div.removeChild( marginDiv );
 
 				return ret;
 			}