add code to drop old databases

Summary: these existed once, are no more, and don't get cleaned up in the current code path

Test Plan: storage destroy --dryrun -- noted the correct database names

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Maniphest Tasks: T2237

Differential Revision: https://secure.phabricator.com/D4329