Remove plain-text file view of Diffusion files.

Summary:
fixes T11792.
There's no good reason any more to have this option, so just drop it.

Test Plan: Load a file, toggle remaining "blame" button. Load search results page and an image too, which are serviced by the same controller.

Reviewers: chad, Blessed Reviewers, epriestley

Reviewed By: Blessed Reviewers, epriestley

Subscribers: Korvin

Maniphest Tasks: T11792

Differential Revision: https://secure.phabricator.com/D16833