Simplify unknown domain nginx configuration

Summary: Avoid a rewrite, as we always serve the same unknown.html file

Test Plan:

• browse a domain pointing to the IP but not configured
• ensure it's still serving unknown.html

Reviewers: dereckson

Reviewed By: dereckson

Subscribers: jokerozen, Sandlayth

Differential Revision: https://devcentral.nasqueron.org/D956