Remove special casing of "arc --version"

Summary: See PHI1765. You can find version information with "arc version". Remove this undocumented special-case.

Test Plan:

• Ran arc --version, no longer got a workflow API exception.
• Searched the documentation for references to arc --version (instead of arc version), found none.

Differential Revision: https://secure.phabricator.com/D21306