remove references to old Jenkins
remove references to old Jenkins

Adriaan confirms it is fully retired.  (the server is still
there, but anything still running is somebody else's)

adriaanm/scala-jenkins-infra,lrytz/scala-jenkins-infra,adriaanm/scala-jenkins-infra,retronym/scala-jenkins-infra,scala/scala-jenkins-infra,adriaanm/scala-jenkins-infra,lrytz/scala-jenkins-infra,retronym/scala-jenkins-infra,lrytz/scala-jenkins-infra,scala/scala-jenkins-infra,retronym/scala-jenkins-infra