I'm facing the same issue, removed trays that now show as failed in Device Manager and Maintenance Utility. Parity Group can't be removed because of failed or blocked parts.
The help in maintenance utility talks about "forcibly remove drives" checkbox which shows up when accessed through Management Console PC 10.0.0.x network. However I can't get this checkbox to show. I'm not sure, but if you can get it you may be able to forcibly delete the parity Group. ( Needs confirmation from experts here)