drivers: counter: stm32: remove deprecated clock source Kconfigs

These clock selection Kconfigs should have been deprecated for
more than 2 releases, remove them:

- `CONFIG_COUNTER_RTC_STM32_CLOCK_SRC`
- `CONFIG_COUNTER_RTC_STM32_CLOCK_LSI`
- `CONFIG_COUNTER_RTC_STM32_CLOCK_LSE`

Signed-off-by: Yong Cong Sin <ycsin@meta.com>