dotenv instead of Rails credentials

The deployment is done on a Dokku server. It sets the DATABASE_URL
automatically.

config/environments/production.rb

@@ -53,7 +53,7 @@ Rails.application.configure do
   config.active_job.queue_adapter = :solid_queue
   config.solid_queue.connects_to = { database: { writing: :queue } }
   config.action_mailer.smtp_settings = {
-    address: Rails.application.credentials.smtp.server,
+    address: ENV['SMTP_SERVER'],
     port: 25,
     enable_starttls_auto: false
   }