grubby-bls: unset default entry if is the one being removed
    
    If the default entry is removed, this can't be set as the default anymore
    for zipl. Otherwise the zipl tool will fail since it won't exist an IPL
    section for the one set as the default.
    
    For grub2, if the saved_entry doesn't exist it will fallback to the first
    entry. But still the correct thing to do is to unset the default there.
    
    Resolves: rhbz#1668329
    
    Signed-off-by: Javier Martinez Canillas <javierm@redhat.com>