Support bin/remove destroy Fnnn for files

Summary: Straightforward (this is the one object type we do let you delete from the web UI) implemetation of PhabricatorDestructibleInterface.

Test Plan: Used bin/remove destroy to destory several files. Used --trace to verify they wiped file data.

Reviewers: btrahan, chad

Reviewed By: chad

Subscribers: epriestley

Differential Revision: https://secure.phabricator.com/D10300