drivers/gpio: stm32: Remove redundant AFIO bit for STM32F1

Specific code is present GPIO_DEVICE_INIT_STM32 in
GPIO_DEVICE_INIT_STM32 to handle LL_APB2_GRP1_PERIPH_AFIO bit.
Though, this bit is already included in device tree information,
in clocks property, 'bits' field, which carries for each GPIO,
both LL_APB2_GRP1_PERIPH_GPIOX and LL_APB2_GRP1_PERIPH_AFIO.
Hence, it is already taken into account in
CONFIG_GPIO_STM32_GPIO##__SUFFIX##_CLOCK_BITS and it is redundant
to handle it in GPIO_DEVICE_INIT_STM32 macro.

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