Clean up remaining default column logic for boards

Summary: See D10189. We should never hit this anymore, so clean it up.

Test Plan:

• Reloaded a board, saw everything stay where it was before the change.
• Added a new task to the project, saw it show up in backlog.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Differential Revision: https://secure.phabricator.com/D10200