Remove `state_changed_at_pipeline_id` column from `vulnerability_state_transitions` table

As part of this #391402 (closed), we introduced a new column state_changed_at_pipeline_id in vulnerability_state_transitions table (!112403 (merged)). Then we decided #393597 (comment 1300638494), the pipeline info is not needed any more or can be removed. Currently there is no data in this column, so safe to remove.

Implementation plan:

1. %17.4 ignore vulnerability_statistics.state_changed_at_pipeline_id column.
   • Ignore state_changed_at_pipeline_id column (!164712 - merged) • Adam Cohen • 17.4
2. %17.5 remove vulnerability_statistics.state_changed_at_pipeline_id column.
   • Remove pipeline reference from state transition (!164207 - merged) • Adam Cohen • 17.5
3. %17.6 remove ignore rule for vulnerability_statistics.state_changed_at_pipeline_id column.
   • Remove ignore state_changed_at_pipeline_id rule (!169550 - merged) • Adam Cohen • 17.6