Ensure Runners are unregistered from environment after being fabricated

Speaking with @jarv today he noted that there appears to be many runners on Staging that were fabricated by gitlab-qa:

Indeed looking at the code for tests that fabricate a runner there's looks to be a code path to remove the docker image and the runner from being registered in the environment but only one test appears to be using this. Other tests look to remove the gitlab runner image but not to unregister it from the environment.

For environments like Staging this leaves numerous runners still registered incorrectly. Task is to ensure they are unregistered.