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_plug-klap.json

@@ -6,6 +6,5 @@
         "encryption_type": "KLAP",
         "https": false,
         "login_version": 2
-    },
-    "uses_http": false
+    }
 }