Remove deprecated runner registration flow code from E2E test framework

Runner registration tokens are being deprecated in 17.0 and removed in 18.0: https://docs.gitlab.com/ee/ci/runners/new_creation_workflow.html

This issue is to remove the deprecated feature code from our E2E test framework.

Checklist

• Delete qa/qa/resource/runner_base.rb (or rename qa/qa/resource/user_runners.rb to runner_base.rb?)
• Remove --registration-token related code from qa/qa/service/docker_run/gitlab_runner.rb and qa/spec/service/docker_run/gitlab_runner_spec.rb