First, visit the external call you want to delete. Click "Delete external call" from the drop-down mentioned below.
This will give you the details of Internal events, where this external call associated to. We can't use external call directly in the program right. Once you got the information, then click Cancel. So then go to the Internal events section, and click delete the Internal event.
Now this will give you the dependency list (example: program builder). Then you need to clear the dependency and then try to delete the external call/internal event.
Good workaround idea, I will give it a try.