Remove construction of "author" information from "LastModified" payload in Diffusion

Summary:
Ref T13552. When viewing a directory in Diffusion, we make an Ajax call to get the last commit for each path.

This call currently pulls author information, since an older version of this UI showed author information.

The current UI does not show author information, so this parameter is unused. Delete the code which builds it.

Test Plan: Grepped for 'author' and references to the "pull-lastmodified" behavior. This behavior is invoked in only one place, which never generates an author placeholder.

Maniphest Tasks: T13552

Differential Revision: https://secure.phabricator.com/D21404