remove readonly (Azure#18416)

Author: nimaller

specification/network/resource-manager/Microsoft.Network/stable/2021-05-01/virtualWan.json

@@ -7854,7 +7854,6 @@
         },
         "expressRouteConnections": {
           "type": "array",
-          "readOnly": true,
           "description": "List of ExpressRoute connections to the ExpressRoute gateway.",
           "items": {
             "$ref": "#/definitions/ExpressRouteConnection"