Deactive debian-9 and dune 2.5/2.6 images from rebuild

Summary

The debian-9 images does not build anymore. We could investigate but there is no use of it, since the debian version is completely outdated. We have a working debian-9 image still uploaded and can rely on this. Also the dune 2.5 and dune 2.6 images do not need to be rebuild every day. Nothing changes there and the underlying debian images also do not get updated. This MR removes these images from the gitlab-ci jobs. They can still be build manually.

Closes #29 (closed)