Revert AddFkFromPartitionedCiRunnerManagersToPartitionedCiRunners

Purpose of revert

This MR no-ops and reverts AddFkFromPartitionedCiRunnerManagersToPartitionedCiRunners which caused gitlab-com/gl-infra/production#18792 (closed).

• Revert AddFkFromPartitionedCiRunnerManagersToPa... (#501545 - closed)
• 2024-10-31: Runner verification API returning 500 (gitlab-com/gl-infra/production#18792 - closed)
• Create ci_runner_machines_687967fa8a partitione... (!168131 - merged)

Checklist

• Create an issue to reinstate the merge request and assign it to the author of the reverted merge request.
• If the revert is to resolve a broken 'master' incident, please read through the Responsibilities of the Broken master resolution DRI.
• If the revert involves a database migration, please read through Deleting existing migrations.
• Add the appropriate labels before the MR is created. We can skip CI/CD jobs only if the labels are added before the CI/CD pipeline is created.

Milestone info

• I am reverting something in the current milestone. No changelog is needed, and I've added a ~"regression:*" label.
• I am reverting something in a different milestone. A changelog is needed, and I've removed the ~"regression:*" label.

Related issues and merge requests