After uninstalling rekordbox, delete the rekordbox folder saved to the below locations.
Windows:
C:\Users\yourusername\AppData\Roaming\Pioneer\rekordbox
Mac:
/Users/yourusername/Library/Pioneer/rekordbox
Note that the Windows AppData folder and the macOS Library folder may be hidden. For instructions on how to display hidden files, contact the publisher of the operating system that you are using.