Remove unusual "Created" element from Build Plan curtain UI

Summary: Ref T13088. Build Plans currently have a "Created" date in the right-hand "Curtain" UI, but this is unusual and the creation date is evident from the timeline. It's also not obvious why anyone would care. Remove it for simplicity/consistency. I think this may have just been a placeholder during initial implementation.

Test Plan: Viewed a build plan, no more "Created" element.

Reviewers: amckinley

Reviewed By: amckinley

Subscribers: PHID-OPKG-gm6ozazyms6q6i22gyam

Maniphest Tasks: T13088

Differential Revision: https://secure.phabricator.com/D20216