Remove the ability to disable blame in Diffusion

Summary: Ref T13105. Given that we now load blame with AJAX, it's not clear that there's any benefit to disabling it. This would also interact oddly with the document engine.

Test Plan: Viewed files in Diffusion, no longer saw blame-related options.

Reviewers: mydeveloperday

Reviewed By: mydeveloperday

Maniphest Tasks: T13105

Differential Revision: https://secure.phabricator.com/D19300