Drop runner_machine_id column from ci_builds_metadata

What does this MR do and why?

Describe in detail what your merge request does and why.

This MR drops the runner_machine_id column from ci_builds_metadata table. The data in that column was never used, so it is safe to delete.

The column was ignored in %15.10, so according to the documentation we should drop the column in %15.11

Part of #392472 (closed), #393340 (closed)

Screenshots or screen recordings

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

How to set up and validate locally

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

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.