Remove package_metadata_sync_using_labels feature flag

What does this MR do and why?

Removes the package_metadata_sync_using_labels feature flag, which gated the label-based fast path in the GCP connector for package metadata sync. The flag has been enabled on GitLab.com since 2026-05-13 and default_enabled: true since 19.0 (!235896 (merged)), so the label path is now the permanent behavior.

• Delete config/feature_flags/beta/package_metadata_sync_using_labels.yml
• Remove the Feature.enabled? guard in retrieve_data_from_labels
• Drop the spec contexts and stubs that exercised the flag-disabled path

The listObjects fallback is unaffected. It was never the flag, it is the nil returns inside data_after_from_labels for missing labels, a stale checkpoint, or a GCS error, and those all stay.

I checked Sentry on the gitlabcom project from 2026-05-13 to today: zero events for MissingChunkFileError, the detection signal named in the rollout issue, and zero for the Google::Cloud::Error rescue in the connector, across the full rollout window.

Remaining manual steps after deploy

• /chatops gitlab run feature delete package_metadata_sync_using_labels --dev --pre --staging --staging-ref --production
• Close #599534

References

• Rollout issue: #599534
• Feature issue: #590584 (closed)
• Introduced by: !230816 (merged)
• Default-enabled by: !235896 (merged)

MR acceptance checklist

Evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.