Replaces empty return value with string [(#2709)](GoogleCloudPlatform/python-docs-samples#2709)

* Replaces empty return value with string
Co-authored-by: Leah E. Cole <6719667+leahecole@users.noreply.github.com>

Author: gguuss

samples/api-client/manager/manager.py

@@ -214,12 +214,12 @@ def delete_registry(
     registry_path = client.registry_path(project_id, cloud_region, registry_id)
 
     try:
-        response = client.delete_device_registry(registry_path)
+        client.delete_device_registry(registry_path)
         print('Deleted registry')
-        return response
+        return 'Registry deleted'
     except HttpError:
         print('Error, registry not deleted')
-        return ""
+        return 'Registry not deleted'
     # [END iot_delete_registry]