bug#43693: To remove references to xv as non-free software

[Jean Louis]

> Under the X Window System, if you have the `xv' program, all of the following
> file formats may be displayed externally as images: gif, tiff, xbm, pm, pbm,
> and jpeg.

Even from DEMO file, the references to xv should be removed, as it is
non-free software, it should not be referenced from GNU Hyperbole, and
it is not used much today, there are many other image viewers and all
are free software.

I use:  Editor:      GNU Emacs 28.0.50 (build 15, x86_64-pc-linux-gnu, X 
toolkit, cairo version 1.14.8, Xaw3d scroll bars)

        Hyperbole:   7.1.2
        Sys Type:    x86_64-pc-linux-gnu
        OS Type:     gnu/linux
        Window Sys:  x
        News Reader: Gnus v5.13