Remove the merge_trains_reconcile_silently_merged feature flag

What does this MR do and why?

Removes the merge_trains_reconcile_silently_merged feature flag. It was introduced in !253406 (merged) to reconcile a merge train car whose merge landed but whose post-merge steps failed, instead of aborting the train and orphaning the commit. The flag has been enabled for 100% of actors on GitLab.com since 2026-09-22, after a staged rollout from 2026-09-17.

This removes the flag check, the legacy reconcile_stuck_car_legacy path, the guard in merge!, the flag definition, and the two spec contexts that covered the disabled behavior.

References

• Issue: #598924 (closed)
• Rollout issue: #628710 (closed)
• Introduced in: !253406 (merged)

Screenshots or screen recordings

Not applicable, backend only.

How to set up and validate locally

bin/rspec ee/spec/services/merge_trains/refresh_merge_request_service_spec.rb

MR acceptance checklist

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Closes #628710 (closed)