Remove uniqueness constraint from "sequence" key in project column table

Summary: Fixes T5611. We don't need sequences to be unique, and it makes it a pain to update them.

Test Plan: Dragged some columns around.

Reviewers: chad

Reviewed By: chad

Subscribers: epriestley

Maniphest Tasks: T5611

Differential Revision: https://secure.phabricator.com/D9914