Mirror scheduling: Remove feature flag

What does this MR do?

This MR removes the update_all_mirrors_worker_rescheduling feature flag and code related to the old behavior.

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Performance and testing

The feature has been enabled on GitLab.com with gitlab-com/gl-infra/production#848 (closed).