fix #15127 ActiveJob::DeserializationError thrown

send_devise_notification pre-maturely enqueued the task when the user instance
has not yet been committed into the database, causing a record-not-found in the
other sidekiq process.

devise-async has already been taking care of asynchronous mail sending, we just
need to run it inside queue `mailers` instead of `mailer` to enable it.

app/models/user.rb

@@ -845,11 +845,6 @@ class User < ActiveRecord::Base
                             other.select(:id)])
   end
 
-  # Added according to https://github.com/plataformatec/devise/blob/7df57d5081f9884849ca15e4fde179ef164a575f/README.md#activejob-integration
-  def send_devise_notification(notification, *args)
-    devise_mailer.send(notification, self, *args).deliver_later
-  end
-
   def ensure_external_user_rights
     return unless self.external?
 

config/initializers/devise_async.rb

 Devise::Async.backend = :sidekiq
+Devise::Async.queue = :mailers