Remove leftover Ci::Runner#schedule_runner_version_update

What does this MR do and why?

This MR removes dead code that should have been removed way back in %16.0.

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

n/a

How to set up and validate locally

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

n/a