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.
Reviewed By: amckinley
Maniphest Tasks: T13105
Differential Revision: https://secure.phabricator.com/D19377