merge_into_manifest: remove canceling of pipelines

The canceling was required when a reintegration was necessary before merging. The instant merge afterwards deleted the integration branch, so the pipeline on the branch would fail, because repo cannot checkout the code anymore. With minimal-manifest!126 we started checking out code using refs/pipelines/<id> instead of explicit branch/tag names. This ref still exists when the branch is already deleted (see https://docs.gitlab.com/ce/ci/pipelines/#ref-specs-for-runners), so the pipeline canceling is not necessary anymore.