do not link to unmaintained docker image cleanup app
What does this MR do?
This MR removes a link to a Docker app hosted outside of GitLab.com and which seems unmaintained.
Why was this MR needed?
gitlab-runner-docker-cleanup#21 describes how the Docker app is unmaintained and does not work very well. It also has many alternatives, including one suggested in this same manual. So to remain consistent, keep a single source of truth for cache cleanup.
What's the best way to test this MR?
N/A. Documentation.