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

Author: anarazel

doc/src/sgml/Makefile

@@ -148,14 +148,6 @@ html-stamp: stylesheet.xsl postgres-full.xml $(ALL_IMAGES)
 	cp $(srcdir)/stylesheet.css html/
 	touch $@
 
-htmlhelp: htmlhelp-stamp
-
-htmlhelp-stamp: stylesheet-hh.xsl postgres-full.xml $(ALL_IMAGES)
-	$(XSLTPROC) $(XMLINCLUDE) $(XSLTPROCFLAGS) $(wordlist 1,2,$^)
-	cp $(ALL_IMAGES) htmlhelp/
-	cp $(srcdir)/stylesheet.css htmlhelp/
-	touch $@
-
 # single-page HTML
 postgres.html: stylesheet-html-nochunk.xsl postgres-full.xml $(ALL_IMAGES)
 	$(XSLTPROC) $(XMLINCLUDE) $(XSLTPROCFLAGS) $(XSLTPROC_HTML_FLAGS) -o $@ $(wordlist 1,2,$^)
@@ -307,8 +299,6 @@ clean:
 # generated SGML files
 	rm -f $(GENERATED_SGML)
 	rm -f postgres-full.xml
-# HTML Help
-	rm -rf htmlhelp/ htmlhelp-stamp
 # EPUB
 	rm -f postgres.epub
 # Texinfo

doc/src/sgml/meson.build

@@ -135,17 +135,6 @@ if xsltproc_bin.found()
   installdocs += install_doc_html
 
 
-  htmlhelp = custom_target('htmlhelp',
-    input: ['stylesheet-hh.xsl', postgres_full_xml],
-    output: 'htmlhelp',
-    depfile: 'htmlhelp.d',
-    depends: doc_generated,
-    command: [xsltproc, '--path', '@OUTDIR@', '-o', '@OUTDIR@/', xsltproc_flags, '@INPUT@'],
-    build_by_default: false,
-  )
-  alldocs += htmlhelp
-
-
   # single-page HTML
   postgres_html = custom_target('postgres.html',
     input: ['stylesheet-html-nochunk.xsl', postgres_full_xml],