MDL-70829 core_search: Mark up decorative image properly

* An empty alt text is enough for decorative images. There's no need
  to set a presentation role.

search/templates/form-user-selector-suggestion.mustache

@@ -36,6 +36,6 @@
     }
 }}
 <span>
-    <img height="12" src="{{profileimageurlsmall}}" alt="" role="presentation">
+    <img height="12" src="{{profileimageurlsmall}}" alt="">
     <span>{{fullname}}</span>
 </span>