修复seeder错误；删除第三方的think迁移工具，在extend中重新实现；

extend/phinx/Db/Action/DropTable.php

@@ -0,0 +1,12 @@
+<?php
+
+/**
+ * MIT License
+ * For full license information, please view the LICENSE file that was distributed with this source code.
+ */
+
+namespace Phinx\Db\Action;
+
+class DropTable extends Action
+{
+}