Remove cvs_ingestion_finding_per_occurrence_ref feature flag

Summary

Removes the cvs_ingestion_finding_per_occurrence_ref feature flag, which was introduced in 19.2 and default-off. The per-occurrence-ref code path is now the permanent behavior. The VAC check remains to gate whether build_finding_maps_per_occurrence_ref or build_finding_maps is used.

Spec contexts that tested the flag-disabled path are removed.

References

#581948 (closed)