config: remove duplicated line

The line is duplicated above in the same file.

config/environments/test.rb

@@ -39,7 +39,6 @@ Rails.application.configure do
   # Print deprecation notices to the stderr.
   config.active_support.deprecation = :stderr
 
-  config.action_mailer.delivery_method = :test
   config.action_mailer.default_url_options = { :host => 'localhost:3000' }
   Rails.application.routes.default_url_options = {
     host: 'localhost:3000',