To get rid of it, you´ll need to remove the persistence.xml cached by the IDE. For example (using Netbeans 8 under Windows 8.1):
C:\Users\jMarcel\AppData\Local\NetBeans\Cache\8.0\index\s71\java\14\classes\META-INF\persistence.xml
Hint: in WindowsExplorer, set your directory to C:\Users\yourUser**AppData\Local\NetBeans** and perform a search for the persistence.xml file.