Remove the "you have not specified reviewers" prompt from the arc client
- This prompt is misleading/confusing in the presence of Herald/Owners.
- This prompt is likely of very little value for experienced reviewers.
- When it works, this prompt may be of some value for new reviewers, but getting it wrong is probably more confusing than getting it right is helpful, and there is a more accurate version of the warning in the web UI that new users are likely to see.
- In the long run, this code should not live in the client.
Test Plan: Created this revision without specifying reviewers, probably didn't get prompted.
Reviewed By: chad
Differential Revision: https://secure.phabricator.com/D16139