cpu/kinetis/rtt: remove callback on clear
cpu/kinetis/rtt: remove callback on clear

miri64/RIOT,RIOT-OS/RIOT,OlegHahm/RIOT,kYc0o/RIOT,OlegHahm/RIOT,kaspar030/RIOT,miri64/RIOT,miri64/RIOT,jasonatran/RIOT,RIOT-OS/RIOT,kaspar030/RIOT,kYc0o/RIOT,kYc0o/RIOT,jasonatran/RIOT,RIOT-OS/RIOT,RIOT-OS/RIOT,ant9000/RIOT,miri64/RIOT,ant9000/RIOT,miri64/RIOT,OlegHahm/RIOT,OlegHahm/RIOT,ant9000/RIOT,kYc0o/RIOT,RIOT-OS/RIOT,ant9000/RIOT,jasonatran/RIOT,kaspar030/RIOT,kYc0o/RIOT,jasonatran/RIOT,kaspar030/RIOT,ant9000/RIOT,OlegHahm/RIOT,jasonatran/RIOT,kaspar030/RIOT