The package-builder image dates back to when we only built a small
subset of packages on Jenkins using a pre-built container with all the
required build dependencies.

With the move to the pkg/ci-package-builder> pipeline on GitLab this has
been no longer needed for quite some time, so it's now a good time to
drop the remnants of the already disabled package-builder image.

Signed-off-by: Emanuele Aina <emanuele.aina@collabora.com>