Fix serverless NEG example in net-lb-app-ext (#3049)

modules/net-lb-app-ext/README.md

@@ -576,6 +576,7 @@ module "glb-0" {
         { backend = "neg-0" }
       ]
       health_checks = []
+      port_name     = ""
     }
   }
   # with a single serverless NEG the implied default health check is not needed

tests/modules/net_lb_app_ext/examples/serverless-neg.yaml

@@ -34,7 +34,6 @@ values:
     locality_lb_policy: null
     name: glb-test-0-default
     outlier_detection: []
-    port_name: http
     project: project-id
     protocol: HTTP
     security_policy: null