docs: Remove support for 'htmlhelp' format

docs: Remove support for 'htmlhelp' format

We had partial support for generating documentation suitable for .chm
files. However, we only had wired up generating the input files using
docbook-xsl, not generating an actual .chm file. Nor did we document how to do
so.  Additionally, it was very slow to generate htmlhelp, as we never applied
the docbook-xsl stylesheet performance improvements to htmlhelp.

It doesn't look like there's any interest in the htmlhelp output, so remove
it, instead of spending cycles to finish the support.

Discussion: https://postgr.es/m/20230324165822.wcrj3akllbqquy7u@awork3.anarazel.de