Change SQL to show columns, index to supress dups

[shoma]

When a columns is used in multipule index, this duplicate in DESCRIBE commnad.
It brings a confuse to build the SQL for the table.

+-------------+--------------+-------------+----------------+---------------------+-------------+-----------------+
| COLUMN_NAME | SPANNER_TYPE | IS_NULLABLE | COLUMN_DEFAULT | INDEX_NAME          | INDEX_TYPE  | COLUMN_ORDERING |
+-------------+--------------+-------------+----------------+---------------------+-------------+-----------------+
| UUID        | STRING(64)   | NO          | <null>         | PRIMARY_KEY         | PRIMARY_KEY | ASC             |
| SingerId    | INT64        | NO          | <null>         | BySingerIdAndStatus | INDEX       | ASC             | <---
| SingerId    | INT64        | NO          | <null>         | PRIMARY_KEY         | PRIMARY_KEY | ASC             |

And omit TABLE_CATALOG and TABLE_SCHEMA from show index command. These are always empty.