configuration item tidb_dml_batch_size should not be displayed to users

## Bug Report

Please answer these questions before submitting your issue. Thanks!

```
other configuration items related to batch DML operations, such as enable-batch-dml,tidb_batch_delete,tidb_batch_insert are not displayed to users, tidb_ dml_ batch_ Size should not be displayed to users
```

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

`configuration item tidb_dml_batch_size should not be displayed to users`

### 3. What did you see instead (Required)

`configuration item tidb_dml_batch_size displayed to users`

### 4. What is your TiDB version? (Required)

`https://docs.pingcap.com/zh/tidb/stable/system-variables#tidb_dml_batch_size `