Delete content in a user registration field after X number of days
Are you sure this is the right way to proceed? 
I think it would be better if you use a custom field with 2 properties: the registration code and a timestamp which holds the time until the value is valid.

And every time when you want to use the value, you check the timestamp. Also with this version you could easily set up a cron or advancedqueue job to delete old values.