Debian preseed: Disable security archive for jessie arm64

Debian preseed: Disable security archive for jessie arm64

jessie arm64 is out of security support, see
  https://wiki.debian.org/LTS
and the security archive at
  http://security.debian.org/dists/jessie/updates/main/binary-arm64
is gone.  This causes autoinstallation to fail.  We don't really care
much about the lack of security support, so override the set of `apt
services'.

I have tested that this can once again now install on arm64 on one of
the laxtons.  Because currently all arm64 tests are broken, and this
change affects only arm64, I will force push it right away.

CC: Julien Grall <julien.grall@linaro.org>
CC: Juergen Gross <jgross@suse.com>
Signed-off-by: Ian Jackson <ian.jackson@eu.citrix.com>