drivers/clock_control: stm32 g0, g4, l4, remove get_pllout_frequency()

After replacing STM32_SRC_PLLCLK by the STM32_SRC_PLL_x sources
this function is no longer needed and are therefore removed.

Also, those functions returned a wrong frequency.
They should have used get_pllsrc_frequency() instead of get_pll_source()
to get the input frequency.

Signed-off-by: Thomas Stranger <thomas.stranger@outlook.com>
3 files changed