Remove CouchDB vhost configuration.

Use the full `_design/app/_rewrite` URL for the registry.

Before this change,
   http://localhost:15984/ was the CouchDB REST API,
   http://127.0.0.1:15984/ was the npm registry REST API.

This commit removes the difference between localhost and 127.0.0.1.

The npm registry REST API must be accessed via
  http://{host:port}/_design/app/_rewrite