doc: remove manpage-base-url workaround

doc: remove manpage-base-url workaround

Commit 50d9bbba92 (Documentation: Avoid use of xmlto --stringparam,
2009-12-04) introduced manpage-base-url.xsl because ancient versions of
xmlto did not have --stringparam.

However, that was more than ten years ago, no need for that complexity
anymore, we can just use --stringparam.

Signed-off-by: Felipe Contreras <felipe.contreras@gmail.com>
Acked-by: Todd Zullinger <tmz@pobox.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>