Push release tags to gitlab.com only

In the past we did not have automatic synchronization of the gitlab-workhorse repository between gitlab.com and dev.gitlab.org. For that reason I set up the release script to push to both of these remotes.

We don't do this anymore in other projects and I think we should stop doing it here too. It's annoying if e.g. your personal access token on dev.gitlab.org expired to have the release script fail.