Remove "days fresh" / "days stale" indictator in Differential revision list

Summary:
Ref T10939. I'm not totally opposed to the existence of this element, but I think it's the kind of thing that would never make it upstream today. I think this should just be a T418 custom sort of thing in the long run, not a mainline upstream feature.

Overall, I think this thing is nearly useless and just adds visual clutter. My dashboard is about 100% red. This also sort of teaches users that it's fine to let revisions sit for a couple of days, which isn't what I'd like the UI to teach. Finally, removing it helps the UI feel a little less cluttered after the visually busy changes in D15926.

Test Plan: Grepped for removed config. Viewed revision list.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T10939

Differential Revision: https://secure.phabricator.com/D15927