Update man page to not mention the GRUB config in the ESP anymore
    
    After https://fedoraproject.org/wiki/Changes/UnifyGrubConfig the config in
    ESP is only used as a stub to load the actual GRUB config the in /boot dir.
    
    Don't mention this file in the grubby man page, since users shouldn't even
    be aware of that config file.
    
    Resolves: rhbz#1958458
    
    Signed-off-by: Javier Martinez Canillas <javierm@redhat.com>