Update gitlab_flow.md, Teatro seems to be completely dead, see also...

Update gitlab_flow.md, Teatro seems to be completely dead, see also https://forum.gitlab.com/t/gitlab-flow-documentation-teatro/7774

doc/workflow/gitlab_flow.md

@@ -91,7 +91,6 @@ This workflow where commits only flow downstream ensures that everything has bee
 If you need to cherry-pick a commit with a hotfix it is common to develop it on a feature branch and merge it into master with a merge request, do not delete the feature branch.
 If master is good to go (it should be if you are practicing [continuous delivery](http://martinfowler.com/bliki/ContinuousDelivery.html)) you then merge it to the other branches.
 If this is not possible because more manual testing is required you can send merge requests from the feature branch to the downstream branches.
-An 'extreme' version of environment branches are setting up an environment for each feature branch as done by [Teatro](https://teatro.io/).
 ## Release branches with GitLab flow