Remove 'htmlhelp' documentat format (was meson documentation build open issues)

Hi,

On 2023-03-24 11:59:23 +0100, Peter Eisentraut wrote:
> Another option here is to remove support for htmlhelp.

That might actually be the best path - it certainly doesn't look like anybody
has been actively using it. Or otherwise somebody would have complained about
there not being any instructions on how to actually compile a .chm file. And
perhaps complained that it takes next to forever to build.

I also have the impression that people don't use the .chm stuff much anymore,
but that might just be me not using windows.

Greetings,

Andres