How to remove external identity from cognito user id

I have linked external identity of Facebook with cognito id using below code:

credentialsProvider?.logins = ["graph.facebook.com" : FBSDKAccessToken.currentAccessToken().tokenString]
credentialsProvider?.refresh()

Taken reference from : http://ift.tt/1IaHt7Q. Now I want that if user logout from Facebook then also want to remove Facebook identity from that cognito id, want to keep same cognito id but just want remove that external identity . How to do that?