gitlab-ci: Drop the special bits now folded in main pipeline

We customized some steps from the main pipeline to ensure that the debian/control file gets re-generated on the fly to reflect our customizations like disabling some build flavors.

The customization has been problematic as it can go out of sync as it indeed did with the recent changes in this repository, yielding errors due to the sequence having changed, stages no longer matching, and artifacts being saved in different locations.

With infrastructure/ci-package-builder!55 (merged) the special bits got folded in the main pipeline, so we can drop the customization here.