remove legacy sku S1

Author: poanchen

specification/iotcentral/resource-manager/Microsoft.IoTCentral/stable/2021-06-01/iotcentral.json

@@ -524,7 +524,6 @@
           "description": "The name of the SKU.",
           "enum": [
             "F1",
-            "S1",
             "ST0",
             "ST1",
             "ST2"