db: Remove invalid enterprise_group index from down migration

power_group replaced enterprise_group and has no valid_range column,
so the rollback index creation would fail.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: jhf

migrations/20260203134134_remove_duplicate_valid_range_gist_indices.down.sql

@@ -10,7 +10,6 @@ CREATE INDEX IF NOT EXISTS ix_legal_unit_valid_range ON public.legal_unit USING
 CREATE INDEX IF NOT EXISTS ix_establishment_valid_range ON public.establishment USING gist (valid_range);
 CREATE INDEX IF NOT EXISTS ix_activity_valid_range ON public.activity USING gist (valid_range);
 CREATE INDEX IF NOT EXISTS ix_contact_valid_range ON public.contact USING gist (valid_range);
-CREATE INDEX IF NOT EXISTS ix_enterprise_group_valid_range ON public.enterprise_group USING gist (valid_range);
 CREATE INDEX IF NOT EXISTS ix_location_valid_range ON public.location USING gist (valid_range);
 CREATE INDEX IF NOT EXISTS ix_person_for_unit_valid_range ON public.person_for_unit USING gist (valid_range);
 CREATE INDEX IF NOT EXISTS ix_stat_for_unit_valid_range ON public.stat_for_unit USING gist (valid_range);