1. If you are using the relation dummy field, delete every field of 'relation'
   type then run cron. This is the same as deleting any other field. 
2. You also need to disable and uninstall every other module depending on the
   Relation Endpoints module in the order allowed.
3. Once relation module itself is disabled and uninstalled it marks the
   endpoints field for deletion. You need to run cron to remove the contents
   of the endpoints table. This might require several cron runs. You will see
   on the modules page how relation endpoints can not be uninstalled because
   there are fields using it.
4. Now you can disable and uninstall relation endpoints itself.