Remove Date from Diff hovercard

Summary: Fixes T6184. On a Revision page we don't show the date as an important piece of information, so it's also not likely useful on a Hovercard (and confusing as to what the date means).

Test Plan: Hover over a linked Diff

Reviewers: btrahan, epriestley

Reviewed By: epriestley

Subscribers: epriestley, Korvin

Maniphest Tasks: T6184

Differential Revision: https://secure.phabricator.com/D10579