Remove deprecated code from beta 16 (#2705)

src/Post/Filter/UserFilter.php

@@ -1,21 +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\Post\Filter;
-
-/**
- * @deprecated beta 16, remove beta 17. Use AuthorFilter instead.
- */
-class UserFilter extends AuthorFilter
-{
-    public function getFilterKey(): string
-    {
-        return 'user';
-    }
-}