Correctly remove sent drafts

Summary:
Kavya,

This is a fix for T406. The syncback code would trigger an exception because it wouldn't search the right db column.

Test Plan: Tested that the exception wasn't triggered.

Reviewers: kav-ya

Reviewed By: kav-ya

Projects: #inbox-sync

Maniphest Tasks: T406

Differential Revision: https://review.inboxapp.com/D434