Drop DifferentialDraft storage

Summary: Fixes T12104.

Test Plan: Ran bin/storage upgrade and observed table dun got dropped.

Reviewers: #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: epriestley

Maniphest Tasks: T12104

Differential Revision: https://secure.phabricator.com/D18034