How to remove Completed Kubernetes Jobs or CronJobs?

This is possible from version 1.12 Alpha with ttlSecondsAfterFinished. An example from Clean Up Finished Jobs Automatically:

apiVersion: batch/v1
kind: Job
metadata:
  name: pi-with-ttl
spec:
  ttlSecondsAfterFinished: 100
  template:
    spec:
      containers:
      - name: pi
        image: perl
        command: ["perl",  "-Mbignum=bpi", "-wle", "print bpi(2000)"]
      restartPolicy: Never