Don't include generated documentation in the tarball

Don't include generated documentation in the tarball

Building it unconditionally causes problems for minimal installations
(OSTree), and building it opportunistically means the tarball isn't
guaranteed to contain it, depending who releases libdbus and which
packages they happen to have installed at the time. If this documentation
is important enough that we need to ship it precompiled in tarballs,
we should guarantee it; or if it isn't important enough to justify that,
we should just drop it.

I don't think we really need it in the tarballs at all: most users
will get their libdbus from a binary distribution (in which case I expect
the distribution's dbus maintainers to set appropriate
build-dependencies), and those who build from source can either
install xmlto, read the documentation on our website, or at worst,
read the source XML. (We don't put the Doxygen-generated API reference
HTML in the tarball either, and I haven't heard any complaints.)

Bug: https://bugs.freedesktop.org/show_bug.cgi?id=55426
Reviewed-by: Colin Walters <walters@verbum.org>