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