Remove "--use-commit-message/-C" from "arc diff"

Summary:
Ref T13544. This flag was introduced in D1385 (2012) as part of a workflow which no longer exists. I can't recall anyone ever reporting an issue which involves its use and believe it is likely unused. It's not obvious to me why someone would use it in modern "arc".

(The same goal can be accomplished with "--message-file ...", although this requires more steps.)

Test Plan: Grepped for "use-commit-message" and "getCommitMessageFromCommit", ran "arc diff".

Maniphest Tasks: T13544

Differential Revision: https://secure.phabricator.com/D21296