rtc: stm32: remove __exit annotation on remove callback
The remove function can be called at runtime for a manual 'unbind'
operation and must not be left out from a built-in driver, as kbuild
complains:
`stm32_rtc_remove' referenced in section `.data.stm32_rtc_driver' of drivers/rtc/rtc-stm32.o: defined in discarded section `.exit.text' of drivers/rtc/rtc-stm32.o