Remove checksum argument from orders.download_order()
`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.
diff --git a/planet/clients/orders.py b/planet/clients/orders.py
index 584df8d..e5e8334 100644
--- a/planet/clients/orders.py
+++ b/planet/clients/orders.py
@@ -265,8 +265,7 @@ class OrdersClient:
                              order_id: str,
                              directory: Path = Path('.'),
                              overwrite: bool = False,
-                             progress_bar: bool = False,
-                             checksum: Optional[str] = None) -> List[Path]:
+                             progress_bar: bool = False) -> List[Path]:
         """Download all assets in an order.
 
         Parameters:
['planet/clients/orders.py']