Make uses_http a readonly property of device config (#1449)

`uses_http` will no longer be included in `DeviceConfig.to_dict()`

tests/fixtures/serialization/deviceconfig_camera-aes-https.json

@@ -5,6 +5,5 @@
         "device_family": "SMART.IPCAMERA",
         "encryption_type": "AES",
         "https": true
-    },
-    "uses_http": false
+    }
 }