Fix navigation menu h2 from showing in legacy Vector

Restores code removed in 50d866dc

Bug: T291264
Change-Id: Ibcdcd0a2dc31a0cdcfa570901765207dc729af6e

resources/skins.vector.styles.legacy/layouts/screen.less

@@ -81,6 +81,12 @@ body {
 	height: 5em;
 }
 
+/* Hide, but keep accessible for screen-readers */
+#mw-navigation h2 {
+	position: absolute;
+	top: -9999px;
+}
+
 #mw-head {
 	position: absolute;
 	top: 0;