Remove "Next Step" Differential custom field

Summary: Ref T12027. This is purely a UI hint for new users that I'd like to integrate into "Land Revision" in the future instead.

Test Plan: Grepped for removed class, browsed Differential.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T12027

Differential Revision: https://secure.phabricator.com/D17076