Appgw: Remove 499 status code from custom error page (#24095)

Author: prateek2211

specification/network/resource-manager/Microsoft.Network/stable/2023-02-01/applicationGateway.json

@@ -3753,7 +3753,6 @@
             "HttpStatus404",
             "HttpStatus405",
             "HttpStatus408",
-            "HttpStatus499",
             "HttpStatus500",
             "HttpStatus502",
             "HttpStatus503",