Don't force arc command to use libdisqus

Summary:
After D8242, we won't need to force `disquslib` in the `arc` command so remove that.

Depends on D8242

Test Plan: N/A

Reviewers: dctrwatson

Reviewed By: dctrwatson

Differential Revision: http://phabricator.local.disqus.net/D8249