drivers: clock_control: remove STM32H7RSX unused function

Remove function exported stm32_system_clock_update() defined in
STM32H7RS series clock driver but that is not used and not even declared.
There already exists a CMSIS SystemCoreClockUpdate() function in
STM32 HAL drivers for the exact same purpose one may use if needed.

No functional change.

Signed-off-by: Etienne Carriere <etienne.carriere@st.com>