code: public Observable<String> deleteAsync(String listId) {
          return deleteWithServiceResponseAsync(listId).map(new Func1<ServiceResponse<String>, String>() {
              @Override
              public String call(ServiceResponse<String> response) {
                  return response.body();
              }
          });
      }
docs: Deletes image list with the list Id equal to list Id passed.
  
  @param listId List Id of the image list.
  @throws IllegalArgumentException thrown if parameters fail the validation
  @return the observable to the String object