drivers: clock_control: stm32: Remove unsupported clock configuration

In stm32 clock_control driver, Kconfig symbol CLOCK_STM32_PLL_PREDIV1
in stm32f3 series was set as conditional on the activation of some
specific STM32F3 SoCs that are not supported yet in Zephyr.
Remove this flag.
In order not to loose a rather studious part of code, comment it out
and set a TODO to specify how to re-enable it once it will be
required.

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