Remove redundant column from historical_accounts

The political_party_id column for historical accounts has been
superseded by political_party_ids (to support floor-crossing
polititions)

Original Tracker story: https://www.pivotaltracker.com/story/show/48664417
Merge pull request #529 from alphagov/remove-redundant-column

Remove redundant column from historical_accounts

Cleanup for: https://www.pivotaltracker.com/story/show/48664417 (already shipped)