Remove defunct "download" route in Files pointing to nonexistent controller

Summary:
Depends on D19154. Ref T13094. This controller was removed at some point and this route no longer works.

I plan to add a new download/ route to let us tighten the form-action Content Security Policy.

Test Plan: Grepped for the route and controller, no hits.

Maniphest Tasks: T13094

Differential Revision: https://secure.phabricator.com/D19155