Remove harmful and unneded workflow rules

The job rules already contain

  rules:
  - if: '$CI_PIPELINE_SOURCE == "merge_request_event"'
    when: manual

which should be enough to restrict the job to run only on MR pipelines. OTOH, having the workflow rules messes up the pipeline in Allen (seen in lhcb/Allen!890 (merged))

@chasse do you remember why this workflow:rules exists in the template? I don't even understand some of the rules.