Remove the "you have not specified reviewers" prompt from the arc client

Summary:
Ref T4631. Ref T10939. I don't have any good solutions here; this is perhaps the least-bad one.

• 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.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T4631, T10939

Differential Revision: https://secure.phabricator.com/D16139