stm32: Remove redundant I2C variant Kconfig symbols definitions

I2C_STM32_V1 and I2C_STM32_V2 are Kconfig symbols used to
select STM32 I2C driver variant based on I2C IP available on a
given STM32 SoC.
Being SoC dependent, they are automatically selected in soc/
if CONFIG_I2C is set. Then there is no need to define them at board
level, nor for a specific application like this is done in
some sample or test applications. Remove these definitions when
this is the case.

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