Revert "fix: "Email notification" value in /my/account when empty string users.mail_notification"

This reverts commit 6317b1d.

Author: nishidayuya

app/models/user.rb

@@ -157,10 +157,6 @@ class User < Principal
     end
   end)
 
-  def normalized_mail_notification
-    mail_notification.presence || 'none'
-  end
-
   def set_mail_notification
     self.mail_notification = Setting.default_notification_option if self.mail_notification.blank?
     true

app/views/users/_mail_notifications.html.erb

@@ -3,7 +3,7 @@
 <%= select_tag(
       'user[mail_notification]',
       options_for_select(
-         user_mail_notification_options(@user), @user.normalized_mail_notification),
+         user_mail_notification_options(@user), @user.mail_notification),
       :onchange => 'if (this.value == "selected") {$("#notified-projects").show();} else {$("#notified-projects").hide();}'
      ) %>
 </p>