Question: how to demote a domain controller that no longer exists?
Answer: In the command line, type ntdsutil and press enter. ... 
At the Ntdsutil: prompt, type metadata cleanup. ... 
At the 'metadata cleanup:' prompt, type connections and press Enter. ... 
In 'server connections:', type : ... 
Type 'q' in server connections to quit and press Enter to return to the metadata cleanup prompt.