rtc: stm32: Remove trivial endif comment
rtc: stm32: Remove trivial endif comment

Removed the endif comment as the Kconfig symbol it referenced didn't
exist.

Signed-off-by: Kumar Gala <a5e5248af4cd4f0ed8c515f61d40a6e2db46a66e@linaro.org>

explora26/zephyr,Vudentz/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,Vudentz/zephyr,finikorg/zephyr,Vudentz/zephyr,galak/zephyr,nashif/zephyr,GiulianoFranchetto/zephyr,GiulianoFranchetto/zephyr,galak/zephyr,finikorg/zephyr,GiulianoFranchetto/zephyr,explora26/zephyr,nashif/zephyr,ldts/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,explora26/zephyr,ldts/zephyr,Vudentz/zephyr,finikorg/zephyr,Vudentz/zephyr,ldts/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,ldts/zephyr,explora26/zephyr,galak/zephyr,nashif/zephyr,GiulianoFranchetto/zephyr,Vudentz/zephyr,explora26/zephyr,GiulianoFranchetto/zephyr,nashif/zephyr,ldts/zephyr,nashif/zephyr,finikorg/zephyr