Remove "dateCommitted" field from DifferentialRevision

Summary: Ref T2222. This is obsolete and no longer used. We could deduce it from transactions or commits in modern Phabricator if we wanted it. We may implement a more general mechanism for T4434.

Test Plan: `grep`

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T2222

Differential Revision: https://secure.phabricator.com/D8330