Offizieller Kommentar
@Pablo > Congrats on the purchase!
No, unfortunately the XDJ-RX is a stand-alone device and was not originally intended as a rekordbox controller, it was only added after many users requested that it be added to the lineup of supported devices. You can use rekordbox for free as an extension of the RX over the ProDJ Link (ethernet) connection, allowing you to load tracks to play on the RX from within rekordbox, but if you want to use rekordbox in performance mode with the RX as a controller, you will require either the free 30 day trial or purchase of a license to unlock performance mode.
To unload a track, you can click on the album artwork and it will "eject" the track, otherwise simply load a new track onto the deck.