You are unable to delete the impersonated account yourself, but you can report the account to Facebook for review. In order to do so, go to the Facebook account in question, click the (...) which appears to the left inside the cover photo, and select Report Account. Next, select Report This Profile and select Continue. Facebook will ask several question concerning the account. Respond accordingly and submit in order for Facebook to review. Once reviewed, Facebook will take the appropriate actions to remove the account.