Answer
Delete it through the separate 'Page Configurations' area, specifically its 'Page Group Details' section, rather than trying to delete it directly from within Page Designer itself where the delete icon may not be shown once a group has been used/attached somewhere. You'll need to already know which specific Page Group it belongs to in order to locate and remove it there, since there isn't a direct lookup from the group back to its containing Page Group shown within Page Designer — if that isn't known or remembered, some additional exploration through Page Configurations may be needed first to identify the correct Page Group entry before it can be deleted.