Hide trash bin icon for group "guest_app"

[hurradieweltgehtunter]

Steps to reproduce

1. Go to user administration panel
2. Click on "guest_app in the left sidebar
3. Click on the trashbin icon to delete the group

Expected behaviour

As the "guest_app" group cannot be deleted it should not display the trashbin icon.

Actual behaviour

Trashbin icon shows, giving the user the idea that this group is deletable but it fails with a error message.
This is technically correct but bad and confusing user experience.

[cortho]

For implementation have a look at owncloud/core#28473 (comment) so virtual groups can be detected by
if ($groupObject->getBackend()->implementsActions(GroupInterface::DELETE_GROUP)) { ....

Edit: I think this is rather a core issue than a guests app specific one