Stop building QA docker image in omnibus-gitlab pipelines

Proposal

1. Stop building GitLab QA docker image in omnibus-gitlab(-mirror) pipelines.
2. Consider that image similar to how we see the assets image - just reuse the one being built in gitlab-org/gitlab pipeline.
3. As part of releases, build the image and publish it in gitlab-org/gitlab pipelines.

Why

1. GitLab QA docker image uses NOTHING from the omnibus-gitlab project. The Dockerfile comes from gitlab-org/gitlab project. Time is wasted on cloning the repo and then building it in omnibus-gitlab pipelines. This feels inefficient and unnecessary.
2. We're already skipping this image build in triggers from gitlab-org/gitlab MRs, so we have precedent.