Use morphTo instead of mappedMorphTo

Turns out we don't need MappedMorphTo after all.

src/Core/Notifications/Notification.php

@@ -1,7 +1,6 @@
 <?php namespace Flarum\Core\Notifications;
 
 use Flarum\Core\Model;
-use Flarum\Core\Support\MappedMorphToTrait;
 
 /**
  * Models a notification record in the database.
@@ -21,8 +20,6 @@ use Flarum\Core\Support\MappedMorphToTrait;
  */
 class Notification extends Model
 {
-    use MappedMorphToTrait;
-
     /**
      * {@inheritdoc}
      */
@@ -114,7 +111,7 @@ class Notification extends Model
      */
     public function subject()
     {
-        return $this->mappedMorphTo(static::$subjectModels, 'subject', 'type', 'subject_id');
+        return $this->morphTo('subject', 'subjectModel', 'subject_id');
     }
 
     /**

src/Core/Notifications/NotificationRepository.php

@@ -26,8 +26,7 @@ class NotificationRepository
             ->skip($offset)
             ->take($limit);
 
-        return Notification::with('subject')
-            ->select('notifications.*', 'p.unread_count')
+        return Notification::select('notifications.*', 'p.unread_count')
             ->mergeBindings($primaries->getQuery())
             ->join(app('flarum.db')->raw('('.$primaries->toSql().') p'), 'notifications.id', '=', 'p.id')
             ->latest('time')