clean-stale-jobs

Deletes unbuildable jobs from multibranch pipeline jobs. Runs nightly. This cleanup job runs a Groovy system script and selects all jobs from the Jenkins instance that are of type WorkflowJob and are not buildable. These jobs are deleted then. Normally the GitHub integration should remove stale branches itself, but we had still many jobs that were unbuildable and thus obsolete. This script fixes the issue.