drivers/clock_control: Remove useless CLOCK_STM32_PLL_XTPRE config

CLOCK_STM32_PLL_XTPRE Kconfig symbols was made to differentiate
code between F1 soc variants with XTRE and others.
It appears that specific XTRE code handling is already in place in
LL_PLL_ConfigSystemClock_* functions that are called afterwards.
Since this piece of code is not required anymore, let's remove
the symbol.

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