Remove create_runner_workflow_for_* feature flags

What does this MR do and why?

Remove create_runner_workflow_for_* feature flags

Changelog: other

Closes #415447 (closed)

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

Verify that you can still create a runner in http://gdk.test:3000/admin/runners (no need to do anything on the command line from the on-screen instructions), and that the registration token "kebab" menu is present:

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.