good answer, dcpromo is supposed to remove the dc. This is not always the case. If dcpromo doesn't completely remove then you can use (Ntdsutil.exe) (metadata cleanup)
The directory service maintains various metadata for each server in the forest. Normally, dc's are created by means of using the dcpromo and they are removed by using the same tool.
Promotion and demotion are designed to correctly clean up the appropriate metadata. In the directory, however, you might have domain controllers that were removed incorrectly. In this case, their metadata is not cleaned up.