boards: nucleo_f302r8: Update to default configuration guidelines
boards: nucleo_f302r8: Update to default configuration guidelines

Update nucleo_f302r8 to fit with default configuration guidelines:
- remove reference to arduino_serial as Ardiuno D1/D0 are actually
mapped on PA2/PA3 which are already used for ST_Link com port,
hence not available by default for arduino serial

Signed-off-by: Erwan Gouriou <de4d967452b83bb0e37525652734e7d9163b6a4c@linaro.org>

ldts/zephyr,zephyrproject-rtos/zephyr,Vudentz/zephyr,Vudentz/zephyr,galak/zephyr,nashif/zephyr,ldts/zephyr,GiulianoFranchetto/zephyr,GiulianoFranchetto/zephyr,explora26/zephyr,explora26/zephyr,zephyrproject-rtos/zephyr,ldts/zephyr,explora26/zephyr,Vudentz/zephyr,zephyrproject-rtos/zephyr,nashif/zephyr,GiulianoFranchetto/zephyr,ldts/zephyr,GiulianoFranchetto/zephyr,zephyrproject-rtos/zephyr,Vudentz/zephyr,Vudentz/zephyr,explora26/zephyr,nashif/zephyr,Vudentz/zephyr,finikorg/zephyr,nashif/zephyr,finikorg/zephyr,ldts/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,GiulianoFranchetto/zephyr,finikorg/zephyr,galak/zephyr,galak/zephyr,finikorg/zephyr,galak/zephyr,nashif/zephyr,galak/zephyr,explora26/zephyr