Use new extenders (#36)

extensions/subscriptions/src/Listener/AddDiscussionSubscriptionAttribute.php

@@ -1,24 +0,0 @@
-<?php
-
-/*
- * This file is part of Flarum.
- *
- * For detailed copyright and license information, please view the
- * LICENSE file that was distributed with this source code.
- */
-
-namespace Flarum\Subscriptions\Listener;
-
-use Flarum\Api\Event\Serializing;
-use Flarum\Api\Serializer\DiscussionSerializer;
-
-class AddDiscussionSubscriptionAttribute
-{
-    public function handle(Serializing $event)
-    {
-        if ($event->isSerializer(DiscussionSerializer::class)
-            && ($state = $event->model->state)) {
-            $event->attributes['subscription'] = $state->subscription ?: false;
-        }
-    }
-}