Remove pipeline reference from state transition

What does this MR do and why?

Removes the vulnerability_state_transitions.state_changed_at_pipeline_id column that was added in Add pipeline reference in vulnerability state t... (!112403 - merged) • Subashis Chakraborty • 15.10.

While working on Create data migration to automatically resolve ... (#444926 - closed) • Adam Cohen • 17.4 • Needs attention and investigating the results of the migration, I was trying to figure out why the state of some vulnerabilities was being reverted from resolved back to detected. I noticed the vulnerability_state_transitions.state_changed_at_pipeline_id column, however, this column was empty for all database records. This was confusing, because I thought that if a vulnerability changes from resolved to detected as a result of a pipeline, then the vulnerability_state_transitions.state_changed_at_pipeline_id should be recorded, after all, that's the entire purpose of the column.

I thought perhaps this was a bug in the code, so I investigated further, and then finally noticed that the column should be removed as part of Remove `state_changed_at_pipeline_id` column fr... (#393934 - closed) • Adam Cohen • 17.6.

In order to prevent anyone else from spending time investigating this behaviour in the future, I think it's best to remove this column now, which is what this MR does.

Related to #393934 (closed)