[MINOR] Remove unnecessary check in MLSerDe

## What changes were proposed in this pull request?
1, remove unnecessary `require()`, because it will make following check useless.
2, update the error msg.

## How was this patch tested?
no test

Author: Zheng RuiFeng <ruifengz@foxmail.com>

Closes #14972 from zhengruifeng/del_unnecessary_check.