variants: remove the ia64 build variant

we no longer have access to ia64 hardware for building sie-dns-sensor
packages and it is unlikely we will need to continue supporting this
platform, so remove the ia64 build variant.
variants: debian 6.0 -> debian 7

debian 7 'wheezy' has now been 'stable' for almost a year, so let's
switch from 'oldstable' to 'stable' for our debian package builds.