drivers: clock_control: Remove unused CLOCK_STM32_PLL2_* symbols

CLOCK_STM32_PLL2_MULTIPLIER and CLOCK_STM32_PLL2_PREDIV2 were added in
commit e1a90583 ("drivers: clock_control: provide LL based driver to
stm32f1xx series"). They have never been used.

Found with a script.

Signed-off-by: Ulf Magnusson <Ulf.Magnusson@nordicsemi.no>