Delete PRI, UNI constraint in create_table_as marco

[Daemonxiao]

What problem does this PR solve?

CREATE TABLE AS SELECT should not copy PRI and UNI constraints.
For more information about this SQL, see the MySQL document.
It says that:

CREATE TABLE ... SELECT does not automatically create any indexes for you. This is done intentionally to make the statement as flexible as possible. If you want to have indexes in the created table, you should specify these before the SELECT statement:
For CREATE TABLE ... SELECT, the destination table does not preserve information about whether columns in the selected-from table are generated columns. The SELECT part of the statement cannot assign values to generated columns in the destination table.
For CREATE TABLE ... SELECT, the destination table does preserve expression default values from the original table.

So we need to add DEFAULT message in tidb__create_table_as marco.

What is changed and how does it work?

Delete PRI, UNI in tidb__create_table_as marco.

Check List

• unit test
• integration test
• modify doc
• change source code