Put HTML documentation in proper directory.
Patch by Chris Mayo, https://bugs.gentoo.org/show_bug.cgi?id=296112

--- a/doc/Makefile.am
+++ b/doc/Makefile.am
@@ -182,8 +182,6 @@
 # Install (automake)
 #
 
-htmldir = $(datadir)/doc/docbook2X
-
 man_MANS = $(MAN_FILES)
 html_DATA = $(HTML_FILES)
 info_TEXINFOS = docbook2X.texi