Only get posts with registered types.

This is so that if an extension adds a post type, and the database gets
populated with posts of that type, but then if the extension is
disabled, we wouldn’t want those posts to display because we would have
no knowledge about how to deal with/render them.

src/Core/Models/Discussion.php

@@ -68,7 +68,7 @@ class Discussion extends Model
         static::deleted(function ($discussion) {
             $discussion->raise(new DiscussionWasDeleted($discussion));
 
-            $discussion->posts()->delete();
+            $discussion->posts()->allTypes()->delete();
             $discussion->readers()->detach();
         });
     }