boards/nrf6310: Remove unused UART_x_EN macro
boards/nrf6310: Remove unused UART_x_EN macro

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