Remove Differential "objectives" UI

Summary: Ref T12733. Completely removes the objectives UI.

Test Plan:

• Grepped for objective, etc.
• Browsed revisions, no JS errors / broken stuff.
• (If I missed anything, it's likely to turn up in followup changes.)

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T12733

Differential Revision: https://secure.phabricator.com/D18043