ContainerServiceVMSize enumeration contains VMSize not supported by AKS agent pool

The API spec definition that enumerates the VM sizes for AKS agent pools https://github.com/Azure/azure-rest-api-specs/blob/master/specification/containerservice/resource-manager/Microsoft.ContainerService/stable/2019-08-01/managedClusters.json#L1098
contain VM Size that is not valid for AKS agent pool.

According to https://docs.microsoft.com/en-us/azure/aks/quotas-skus-regions#restricted-vm-sizes the `Standard_A1_v2` VM Size is not allowed for AKS agent pools however https://github.com/Azure/azure-rest-api-specs/blob/master/specification/containerservice/resource-manager/Microsoft.ContainerService/stable/2019-08-01/managedClusters.json#L1098 contains it.

This causes AKS cluster creations through API to fail with `containerservice.ManagedClustersClient#CreateOrUpdate: Failure sending request: StatusCode=400 -- Original Error: Code="BadRequest" Message="The VM SKU chosen for this cluster `Standard_A1_v2` does not have enough CPU/memory to run as an AKS node. Please see https://aka.ms/aks/restricted-skus for more details.`

cc @lpuskas @ArcturusZhang

Start with specification/containerservice/resource-manager/Microsoft.ContainerService/stable/2019-08-01/managedClusters.json around line 1098 and compare the VM size enumeration with the linked AKS restricted VM sizes documentation. Confirm the invalid Standard_A1_v2 entry and validate that the enumeration no longer advertises it.

Written by the indexing model from the issue text.