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

src/configuration/ArcanistConfiguration.php

@@ -26,9 +26,6 @@ class ArcanistConfiguration extends Phobject {
       // Special-case "arc --help" to behave like "arc help" instead of telling
       // you to type "arc help" without being helpful.
       $command = 'help';
-    } else if ($command == '--version') {
-      // Special-case "arc --version" to behave like "arc version".
-      $command = 'version';
     }
 
     $workflow = idx($this->buildAllWorkflows(), $command);