Remove the "authored" subheader from commits

Summary:
Ref T13552. I'm trying to reduce the number of direct callers to commit authorship metadata. This header seems low-value enough to simply remove; this information is shown more clearly and prominently in the "Provenance" UI.

In particular, commits have multiple dates (authored, committed, pushed) but this header shows only one. It currently shows the author identity and the commit date, which isn't entirely correct. And it potentially uses an "Identity" as a timeline actor, which is conceptually fine but not entirely firm ground.

Test Plan: Viewed a commit, saw no more subheader.

Maniphest Tasks: T13552

Differential Revision: https://secure.phabricator.com/D21406