show create table will output underscore charset constraint expr

## Bug Report

Please answer these questions before submitting your issue. Thanks!

### 1. Minimal reproduce step (Required)

As mentioned in the title, which will result in schema export fail.

### 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