boards/chronos: remove UART config
boards/chronos: remove UART config

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