boards/z1: Remove unused UART_x_EN macro
boards/z1: Remove unused UART_x_EN macro

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