[QA] Stop Service::Runner trying to remove a non-existent runner

The following discussion from !220 (merged) should be addressed:

• @mlapierre started a discussion:

  https://gitlab.com/gitlab-org/gitlab-qa/-/jobs/161645087

  An error occurred in an `after(:context)` hook.
  Failure/Error: Service::Runner.new(@runner_name).remove!
  
  QA::Service::Shellout::CommandError:
    Command `docker rm -f qa-runner-1550165298` failed!

This is a problem in quarantine jobs. clone_using_deploy_key_spec.rb starts a runner in before(:all), but that will be skipped if the whole context is skipped. Then the after hook tries to remove a non-existent runner.

We could skip after hooks when quarantined before hooks are skipped, but that might skip some necessary cleanup (which is why RSpec's skip feature doesn't skip after hooks when before hooks are skipped).

It might be better to update Service::Runner to not try to remove a non-existent runner.