Remove checksum argument from orders.download_order()

[kevinlacaille]

orders.download_order() has a checksum argument, which is never used, which may be misleading to a user if they mark checksum=True.

• In the SDK, the checksum is done in a separate call (e.g., orders.download_order() then orders.validate_checksum()).
• In the CLI, the checksum is just a flag baked into the CLI's functionality.

To do: Remove checksum argument from orders.download_order() to avoid confusion.