Revert "Merge branch 'jk/ci-use-system-asciidoctor' into next"

This reverts commit e7621b4bad, reversing
changes made to d0859510f8.

ci/install-dependencies.sh

@@ -166,6 +166,7 @@ sparse)
 Documentation)
 	sudo apt-get -q -y install asciidoc xmlto docbook-xsl-ns make ruby
 
+	test -n "$ALREADY_HAVE_ASCIIDOCTOR" ||
 	sudo gem install --version 1.5.8 asciidoctor
 	sudo gem install concurrent-ruby
 	;;