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. Select the one you want to change, and then do "Get-ClusterGroup <AAG_NAME> | Get-ClusterResource", and it will list all of your AAG's Resources. Select the one you want to remove, and use that in place of "<AAG_NAME>_<IP_TO_REMOVE>" in the command above.