[SPARK-15722][SQL] Disallow specifying schema in CTAS statement

## What changes were proposed in this pull request?

As of this patch, the following throws an exception because the schemas may not match:
```
CREATE TABLE students (age INT, name STRING) AS SELECT * FROM boxes
```
but this is OK:
```
CREATE TABLE students AS SELECT * FROM boxes
```

## How was this patch tested?

SQLQuerySuite, HiveDDLCommandSuite

Author: Andrew Or <andrew@databricks.com>

Closes #13490 from andrewor14/ctas-no-column.