fix bugs

migrations/mysql/02108_user_referral.sql

@@ -1,4 +1,3 @@
--- migrate:up
 ALTER TABLE `user`
     ADD COLUMN `referral_percentage` TINYINT UNSIGNED NOT NULL DEFAULT 0
       COMMENT 'Referral Percentage'
@@ -7,7 +6,3 @@ ALTER TABLE `user`
       COMMENT 'Only First Purchase'
       AFTER `referral_percentage`;
 
--- migrate:down
-ALTER TABLE `user`
-DROP COLUMN `referral_percentage`,
-DROP COLUMN `only_first_purchase`;