Remove "arc diff --no-diff"

Summary: Ref T12996. See PHI84. This is a remnant from long ago that no longer functions.

Test Plan: Created this revision. Grepped for no-diff. Grepped for diff-result.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T12996

Differential Revision: https://secure.phabricator.com/D18640