[AzureSearch] Make skillsetCount optional for 2020-GA API service stats (#9966)

* make skillsetCount optional for 2020-GA API

* remove extra comma

Co-authored-by: Austin Drefke <[email protected]>

Author: jadrefke

specification/search/data-plane/Azure.Search/preview/2020-06-30/searchservice.json

@@ -8501,8 +8501,7 @@
         "indexersCount",
         "dataSourcesCount",
         "storageSize",
-        "synonymMaps",
-        "skillsetCount"
+        "synonymMaps"
       ],
       "description": "Represents service-level resource counters and quotas."
     },