show create table will output underscore charset constraint expr

[AilinKid]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

As mentioned in the title

2. What did you expect to see? (Required)

No underscore expr in constraint check or we should have an option for this

3. What did you see instead (Required)

Always output underscore contraint check expr

4. What is your TiDB version? (Required)

Master