drop 90_persistent script
    
    It is undocumented.
    
    It haven't been upstreamed.
    
    Upstream has 40_custom and 41_custom for the same purpose.
    
    It violates layering and code structure by guessing which (optional!) output
    file has been specified for grub2-mkconfig ... and makes the wrong guess for
    grub2-efi.