bugfix: removed double creation of unique index

[fmazan]

Changes in this PR

• removing of double unique index creation, as it is already created in playhouse - https://github.com/coleifer/peewee/blame/f107a90897a21c9e4ef1aa43bd61a54ceb5e943f/playhouse/migrate.py#L345
• without this commit the behavior is:

  2023-08-04 11:53:51,517 20748 9968 N/A INFO peewee_migrate Migrate "123_add_uuid"
  2023-08-04 11:53:51,517 20748 9968 N/A INFO peewee_migrate add_column ('some_table', 'uuid', <SomeField: SomeModel.uuid>)
  2023-08-04 11:53:51,517 20748 9968 N/A DEBUG peewee ('ALTER TABLE `some_table` ADD COLUMN `uuid` BINARY(16)', [])
  2023-08-04 11:53:51,521 20748 9968 N/A DEBUG peewee ('CREATE UNIQUE INDEX `some_table_uuid` ON `some_table` (`uuid`)', [])
  2023-08-04 11:53:51,528 20748 9968 N/A INFO peewee_migrate add_index ('some_table', ('uuid',))
  2023-08-04 11:53:51,528 20748 9968 N/A DEBUG peewee ('CREATE UNIQUE INDEX `some_table_uuid` ON `some_table` (`uuid`)', [])
  ...
  pymysql.err.OperationalError: (1061, "Duplicate key name 'some_table_uuid'")

cc/ @klen

[klen]

@fmazan Thank you!