Avoid creating or updating runner managers when not necessary

In the gitlab-com/gl-infra/production#18792 (closed) incident, it became clear that some REST calls (in this case DELETE /api/:version/runners) unnecessarily failed because they tried to create a runner manager for a runner that is being deleted. This is redundant and wasteful, so we should pass ensure_runner_manager: false, update_contacted_at: false to API::Ci::Helpers::Runner#authenticate_runner! from the DELETE /api/:version/runners endpoint.