Remove deprecated prefix option from JsRoutes

With the latest release, the default_url_options (imported and configured through Rails) is sufficient.

config/initializers/js_routes.rb

@@ -2,6 +2,5 @@
 
 JsRoutes.setup do |config|
   config.documentation = false
-  config.prefix = Rails.application.config.relative_url_root
   config.url_links = true
 end