add back flatten flag for delegated subnet because its parameters are nested

specification/dnc/resource-manager/Microsoft.DelegatedNetwork/stable/2021-03-15/delegatedSubnets.json

@@ -388,6 +388,7 @@
       ],
       "properties": {
         "properties": {
+          "x-ms-client-flatten": true,
           "$ref": "#/definitions/DelegatedSubnetProperties",
           "description": "Properties of the provision operation request."
         }