UI: just show confirmation for challenge cleanup

Previously required to check result which could be null

src/Certify.UI.Shared/Controls/ManagedCertificate/AdvancedOptions.xaml.cs

@@ -255,7 +255,8 @@ private async void ChallengeCleanup_Click(object sender, RoutedEventArgs e)
         {
             var result = await AppViewModel.Current.PerformChallengeCleanup(ItemViewModel.SelectedItem);
 
-            AppViewModel.Current.ShowNotification(result.FirstOrDefault().Message);
+            AppViewModel.Current.ShowNotification("Challenge Cleanup Completed");
+
         }
     }
 }