samples: boards: stm32: no need to configure button pin

The gpio pin that will be used as a source for PWR wake-up pins does not
actually need to be configured as input, hence input_gpio_keys driver does
not need to be involved. As far as GPIO controller is concerned, the pin
can be in analog state.

Also, it's clearer & safer to call gpio_pin_configure_dt directly in the
application with the STM32_GPIO_WKUP flag, just before calling Poweroff,
instead of relying on a driver, like input_gpio_keys to do it just after
boot behind the scenes, & risk not having the correct wake-up pins config
when Poweroff is finally called.

Signed-off-by: Abderrahmane Jarmouni <abderrahmane.jarmouni-ext@st.com>