Convert a manual query against external accounts into a modern Query

Summary: Depends on D20108. Ref T6703. Update this outdated callsite to a more modern appraoch.

Test Plan: Destroyed a user with bin/remove destroy.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T6703

Differential Revision: https://secure.phabricator.com/D20109