When upstream transition a release from stable to unmaintained status, the stable branch is removed and the unmaintained one is created.
But DLRN sticks to stable branch locally and thus does not build commits from the unmaintained branch.
We need to remove branch locally once it's removed upstream.

[x] propose DLRN fix
[x] DLRN fix reviewed and merged in master
[x] update trunk builders with latest DLRN codebase