[ReleasePR alertsmanagement] Remove Preview PrometheusRuleGroups resource type (#2100)

Create to sync Azure/azure-rest-api-specs#17060
[ReCreate this PR](https://github.com/azure-resource-manager-schemas/compare/main...AzureSDKAutomation:sdkAuto/alertsmanagement?expand=1)

Author: msftbot[bot]

schemas/2021-07-22-preview/Microsoft.AlertsManagement.json

@@ -70,9 +70,6 @@
         {
           "$ref": "https://schema.management.azure.com/schemas/2021-04-01/Microsoft.AlertsManagement.json#/resourceDefinitions/smartDetectorAlertRules"
         },
-        {
-          "$ref": "https://schema.management.azure.com/schemas/2021-07-22-preview/Microsoft.AlertsManagement.json#/resourceDefinitions/prometheusRuleGroups"
-        },
         {
           "$ref": "https://schema.management.azure.com/schemas/2021-08-08-preview/Microsoft.AlertsManagement.json#/resourceDefinitions/actionRules"
         },