Drop intended support for "--anonymous" from Arcanist Toolsets

Summary:
Currently, modern "arc" workflows accept and parse "--anonymous" but don't do anything with it.

I intend to move away from anonymous workflows, which only really supported a tiny subset of unusual workflows on open source installs but added significant complexity to login/auth behavior.

Drop support for this flag.

Test Plan: Grepped for "anonymous", didn't turn up any relevant hits.

Differential Revision: https://secure.phabricator.com/D21386