remove unneeded migration

extensions/mentions/migrations/2022_10_21_000001_alter_created_at_set_current_timestamp_on_post_mentions_group.php

@@ -1,23 +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.
- */
-
-use Illuminate\Database\Schema\Builder;
-
-return [
-    'up' => function (Builder $schema) {
-        // do this manually because dbal doesn't recognize timestamp columns
-        $connection = $schema->getConnection();
-        $prefix = $connection->getTablePrefix();
-        $connection->statement("ALTER TABLE `${prefix}post_mentions_group` MODIFY created_at TIMESTAMP NULL DEFAULT CURRENT_TIMESTAMP");
-    },
-
-    'down' => function (Builder $schema) {
-        // Nothing to do.
-    }
-];