Remove reject_orphaned_runners feature flag

What does this MR do and why?

This MR removes the reject_orphaned_runners which was introduced this milestone for .com de-risking. There's no changelog because the FF is being removed in the same milestone.

We can now remove the FF as it was confirmed to work as expected in production, since Friday 17:21. The orphaned runners that were contacting ~70K times/h and receiving HTTP 422 are now receiving HTTP 403.

References

Please include cross links to any resources that are relevant to this MR. This will give reviewers and future readers helpful context to give an efficient review of the changes introduced.

• Remove code that ensures that a runner exists i... (#516862 - closed)
• [Feature flag] Rollout of `reject_orphaned_runn... (#516929 - closed)

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