d/iptables.manpages: install nfnl_osf.8 from upstream tarball

I deleted the Debian patch to add our custom manpage which is included upstream
in the last release. Instruct debhelper to pick the upstream one.

Closes: #889649
Reported-by: Daniel Baumann <daniel.baumann@progress-linux.org>
Signed-off-by: Arturo Borrero Gonzalez <arturo@debian.org>