Completed task can't be deleted because they are not in the count of Show completed tasks (0)
Looks like there is a problem with completed task that have been indented to the right and then completed. They are not in the list of the count of Show completed tasks (0) so I can't delete them of the proyect to go to the completed count on the top right of the page.
sub-tasks always stay on your task list for as long as their parent task hasn't been completed.
However, you can force a sub-task to move to the "completed tasks" list if you hold down the SHIFT key while checking it off.