Remove deprecated ci_runners columns

What does this MR do and why?

This MR removes deprecated ci_runners columns, after they were marked as ignored in %17.0 with !151343 (closed).

This MR represents the https://docs.gitlab.com/ee/development/database/avoiding_downtime_in_migrations.html#dropping-the-column-release-m1 phase of removing the columns.

NOTE: The check-migrations job is failing but that is a false positive, as explained in the docs.

Changelog: removed

Part of #458721 (closed)

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