Disable Nginx 404 handler for static assets

This extra production-minded rule prevents some useful plugins from working properly, such as [Dependency Minification](https://github.com/x-team/wp-dependency-minification) and [Media Placeholders](https://github.com/x-team/wp-media-placeholders), since they rely on the WordPress handling requests to JS, CSS, and images. The Nginx config goes above-and-beyond the default equivalent `.htaccess` that WordPress generates, and I think @TheLastCicada was in agreement today at WCPDX that this can be removed.