Prevent the admin user from being deleted
    
    admin is required for trust operations
    
    Note that testing for removing the last member is now
    irrelevant because admin must always exist so the test
    for it was removed, but the code check remains. It is done
    after the protected member check.
    
    Fixes: https://pagure.io/freeipa/issue/8878
    
    Signed-off-by: Rob Crittenden <rcritten@redhat.com>
    Reviewed-By: Alexander Bokovoy <abokovoy@redhat.com>