Remove lib/demo.g

[ssiccha]

The functions in these file are unused, undocumented and a proper
demo functionality is now available via the browse package.

[olexandr-konovalov]

It might be better to start with opening an issue or a mailing list question and ask whether anyone uses it.

For example, I've used it in the past for demos, and have a tweaked version of it in
https://github.com/gap-packages/scscp/blob/master/demo/rundemo.g.

Today, I will be most likely using Jupyter interface for demos, or look at NCurses.Demo from Browse (thanks for drawing attention to it, I didn't know that such functionality exists). However, I do think that lib/demo.g may still be useful for those who would like to run teaching demos and can not use Jupyter or Browse package. While these packages must be properly installed, with all dependencies satisfied, the lib/demo.g file can be used straightforwardly.

However, I am not using ReadVerbose and I'm fine if it will disappear. Also, the tweak in SCSCP's demo mode was to use coloured prompt - maybe it could be backported to lib/demo.g and removed from SCSCP (or go to Utils package)... And the demo is untested, because it requires manual interaction.

Alternatively, one still could merge this PR, and I will update SCSCP demo mode to deal with defining last* and time before running a demo.

[ssiccha]

I think the sensible thing to do would then be to document this function (and make sure that the documentation also points to the more sophisticated demo feature in browse). And yes, ideally there shouldn't be two similar versions of the same file. I've opened issue gap-packages/scscp#24 on SCSCP. :)