Remove visual_gitlab_integration CI job

We recently had to allow the visual_gitlab_integration job to fail when it started failing for no obvious reason.

While this job has been useful to prevent some CSS leak issues in the past, it doesn't seem like we've had such issues lately thanks to some CSS structure changes.

We feel like the cost of fixing the job might be greater than the resulting benefit. In consequence, let's remove this job altogether!

Let's also remove the Include GitLab CSS checkbox which produces the same results as the CI job.

Related internal discussion: https://gitlab.slack.com/archives/CH9QG9TAQ/p1606510558271600