soc: stm32: Remove direct COUNTER_RTC_STM32 to COUNTER dependency

COUNTER_RTC_STM32 used to be enabled directly based on COUNTER status.
This should not be the case anymore as we should first check
DT_HAS_ST_STM32_RTC_ENABLED status (which is already done in
drivers/counter/Kconfig.stm32_rtc).

Remove these 3 lines that are not correct anymore.

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