Remove the "--less-context" flag from "arc diff"

Summary: Ref T13544. This is an obscure flag and almost never useful. You can accomplish the same goal, roughly, with "git diff | arc diff --raw -". See also PHI675. See also T13338.

Test Plan: Grepped for "less-context", created this revision.

Maniphest Tasks: T13544

Differential Revision: https://secure.phabricator.com/D21301