DEV: Don't return navigation item from addNavigationBarItem

No plugin or theme that I could find used the return value. Not
providing this to plugins affords us leeway to change the internals.

app/assets/javascripts/discourse/lib/plugin-api.js.es6

@@ -785,7 +785,7 @@ class PluginApi {
         };
       }
 
-      return addNavItem(item);
+      addNavItem(item);
     }
   }