deploy_gitlab_ci: revert trigger pipeline on integration branch

Explicitly triggering a pipeline in the target repository does not work
easily from this place because we would have to distinguish between the
manifest (which uses branch pipelines) and the sub-projects (which use
merge request pipelines). This separation should not be hard-coded here,
so we move the decision whether or not to run a pipeline to the
.gitlab-ci.yml file of the target. Reverting these two commits:

63279799
    deploy_gitlab_ci: trigger pipeline only if it has not run before

dfc6204a
    deploy_gitlab_ci: trigger pipeline on integration branch