deb-pkg: remove obsolete -isp option to dpkg-gencontrol

The -isp option has been deprecated, after it became the default
behaviour back in 2006.

Since dpkg 1.17.11, dpkg-gencontrol emits a warning on -isp usage.

References: https://bugs.debian.org/215233


Signed-off-by: Asbjoern Sloth Toennesen <asbjorn@asbjorn.biz>
Signed-off-by: Michal Marek <mmarek@suse.cz>