Remove "--ignore-unsound-tests" from "arc diff"
Remove "--ignore-unsound-tests" from "arc diff"

Summary: Ref T13544. This flag only disables a warning and should be a prompt default, not a flag.

Test Plan: Grepped for "ignore-unsound-tests", created this revision.

Maniphest Tasks: T13544

Differential Revision: https://secure.phabricator.com/D21303

Khan/arcanist,codelabs-ch/arcanist,Khan/arcanist,phacility/arcanist,codelabs-ch/arcanist,phacility/arcanist,phacility/arcanist,Khan/arcanist,phacility/arcanist,codelabs-ch/arcanist,phacility/arcanist,codelabs-ch/arcanist,Khan/arcanist,codelabs-ch/arcanist