docs: Remove docs for non-existing pxe_unload command
    
    Remove the documentation of the pxe_unload command from the
    docs/grub.texi file.
    
    The pxe_unload command is not implemented in the grub source
    at this time at all. It appears to have been removed in commit
    671a78acb (cleanup pxe and efi network release).
    
    Signed-off-by: Hans Ulrich Niedermann <hun@n-dimensional.de>
    Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>