It's a PowerShell command.... Run it from PowerShell. And whoops, it's actually Get-ClusterGroup <AAG_NAME> | Get-ClusterResource <AAG_NAME>_<IP_TO_REMOVE> | Remove-ClusterResource

Just do this... Run "Get-ClusterGroup", and it will list your cluster groups....

You have to remove it from the Windows Cluster Resource Object: Get-ClusterGroup <WINCLUSTER_NAME> | Get-ClusterResource <AAG_NAME>_<IP_TO_REMOVE> | Remove-ClusterResource