packaging-updates: Stop using the gitlab-update-job branches
With the switch to the external gitlab-ci pipeline definition in the
packaging repositories the debian/buster-gitlab-update-job
branches
are no longer needed since they were only used to host the upstream pull
pipeline definition which it could not reside on the debian/
branches.
Now we can kick the pipeline on the debian/
branches directly using
the external config, so let's do that.