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 <erwan.gouriou@linaro.org>