Remove non-standard outdated CSS

Removed `-moz-osx-font-smoothing: grayscale;` and `-webkit-font-smoothing: antialiased;`, as suggested in woocommerce/storefront#698

assets/css/_mixins.scss

@@ -179,7 +179,6 @@
 	font-variant: normal;
 	text-transform: none;
 	line-height: 1;
-	-webkit-font-smoothing: antialiased;
 	margin: 0;
 	text-indent: 0;
 	position: absolute;