Remove obsoleted "diffusion-browse-file" behavior for coverage

Summary: Ref T13105. After moving Diffusion to DocumnentEngine, this no longer has callers. It will become part of the document behavior.

Test Plan: Grepped for calls to the diffusion-browse-file behavior, found none.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13105

Differential Revision: https://secure.phabricator.com/D19377