Merge Request pipeline variables without introducing MR pipelines

See https://gitlab.hzdr.de/hifis/software/education/gitlab-ci/workshop-materials/-/blob/948b6db86ef98625c8325f4d6ab95b67454f44ec/episodes/04-extending-the-pipeline.md#predefined-variables-for-merge-request-pipelines.

Probably, we should just remove it to not confuse people. It's still available from the GitLab documentation.