Remove ingest_sec_reports_when_sec_jobs_completed feature flag

What does this MR do and why?

This MR removes the ingest_sec_reports_when_sec_jobs_completed feature flag, which has been successfully rolled out to production and is now enabled globally.

Closes #554222

Changes

• Removed the feature flag YAML definition
• Removed the feature flag check in ee/app/models/ee/ci/build.rb - now always publishes JobSecurityScanCompletedEvent when security jobs complete
• Removed the old code path in ee/app/models/ee/ci/pipeline.rb that was executed when the feature flag was disabled
• Updated specs to remove feature flag stubbing and test only the new behavior

How to set up and validate locally

1. Verify that security report ingestion happens when security jobs complete
2. Run the affected specs to ensure they pass

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist.