Remove "arc diff --plan-changes"

Summary: Experimental branch. Ref T2543. To be replaced with "arc diff --draft". Depends on D18737.

Test Plan: Grepped for plan-changes.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T2543

Differential Revision: https://secure.phabricator.com/D18738