Remove `ci_pipeline_process_worker_dedup_until_executed` feature flag

What does this MR do and why?

This MR cleans up the feature flag ci_pipeline_process_worker_dedup_until_executed which was successfully rolled out globally in #397829 (closed) at 2023-05-09 11:00 PST.

We should merge this after a full 24-hour verification period (at 2023-05-10 11:00 PST or later).

Currently, the monitoring results indicate that this feature will likely be stable.

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.

Related to #397829 (closed)