How do I delete migrated files

It depends. If you used Migration Assistant, migrated files are on a separate user, so open System Preferences > Users & Groups, select the user and press - button to delete it.

If you used Setup Assistant (you transferred files while your Mac was setting up), migrated files are on your user, so you must remove them manually

When you delete an user, it's converted onto a disk image on /Users/Deleted Users. To remove them, open Finder, select Go menu > Go to Folder, type /Users/Deleted Users and delete all disk images inside the folder. See > http://support.apple.com/kb/PH6596?viewlocale=en_US&locale=en_US