code: public static void deregister(@Nonnull Item item) {
          ItemDeletion instance = instance();
          if (instance != null) {
              instance.lock.writeLock().lock();
              try {
                  instance.registrations.remove(item);
              } finally {
                  instance.lock.writeLock().unlock();
              }
          }
      }
docs: Deregister the supplied {@link Item} for deletion.
  
  @param item the {@link Item} that was to be deleted and is now either deleted or the delete was aborted.