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

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],

doc/src/sgml/stylesheet-hh.xsl

@@ -1,47 +0,0 @@
-<?xml version='1.0'?>
-<xsl:stylesheet xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
-                version='1.0'>
-
-<xsl:import href="http://docbook.sourceforge.net/release/xsl/current/htmlhelp/htmlhelp.xsl"/>
-<xsl:include href="stylesheet-common.xsl" />
-
-<!-- Parameters -->
-<xsl:param name="htmlhelp.use.hhk" select="'1'"/>
-
-<xsl:param name="base.dir" select="'htmlhelp/'"></xsl:param>
-<xsl:param name="html.stylesheet" select="'stylesheet.css'"></xsl:param>
-<xsl:param name="use.id.as.filename" select="'1'"></xsl:param>
-<xsl:param name="manifest.in.base.dir" select="1"/>
-<xsl:param name="make.valid.html" select="1"></xsl:param>
-<xsl:param name="generate.id.attributes" select="1"></xsl:param>
-<xsl:param name="generate.legalnotice.link" select="1"></xsl:param>
-<xsl:param name="link.mailto.url">pgsql-docs@lists.postgresql.org</xsl:param>
-<xsl:param name="chunker.output.indent" select="'yes'"/>
-<xsl:param name="chunk.quietly" select="1"></xsl:param>
-
-
-<!-- Change display of some elements -->
-
-<xsl:template match="command">
-  <xsl:call-template name="inline.monoseq"/>
-</xsl:template>
-
-<!--
-  Format multiple terms in varlistentry vertically, instead
-  of comma-separated.
- -->
-
-<xsl:template match="varlistentry/term[position()!=last()]">
-  <span class="term">
-    <xsl:call-template name="anchor"/>
-    <xsl:apply-templates/>
-  </span><br/>
-</xsl:template>
-
-
-<!-- strip directory name from image filerefs -->
-<xsl:template match="imagedata/@fileref">
- <xsl:value-of select="substring-after(., '/')"/>
-</xsl:template>
-
-</xsl:stylesheet>