Remove jenkins bootstrapping code

Change-Id: I227abd2b6ad1dfee550c05fda32a97785cc957fe
Reviewed-on: https://forge.frm2.tum.de/review/18669
Tested-by: JenkinsCodeReview <bjoern_pedersen@frm2.tum.de>
Reviewed-by: Enrico Faulhaber <enrico.faulhaber@frm2.tum.de>
Reviewed-by: Bjoern Pedersen <bjoern.pedersen@frm2.tum.de>

ci/Jenkinsfile

@@ -143,7 +143,7 @@ node("master") {
    }
     stage('(re-)generate image') {
        sh '''#!/bin/bash
-        git worktree add tmpmaster refs/changes/61/18661/8
+        git worktree add tmpmaster refs/heads/master
         cd tmpmaster
         docker build --target base --tag secop_base:latest ci
         docker build --target docs --tag secop_docs:latest ci