boards/im880b: remove RTC_NUMOF
boards/im880b: remove RTC_NUMOF

These defines were globally removed in #12673.

kaspar030/RIOT,ant9000/RIOT,jasonatran/RIOT,ant9000/RIOT,ant9000/RIOT,kaspar030/RIOT,ant9000/RIOT,kaspar030/RIOT,kaspar030/RIOT,OlegHahm/RIOT,OlegHahm/RIOT,OTAkeys/RIOT,kYc0o/RIOT,miri64/RIOT,ant9000/RIOT,OTAkeys/RIOT,OlegHahm/RIOT,authmillenon/RIOT,miri64/RIOT,jasonatran/RIOT,RIOT-OS/RIOT,jasonatran/RIOT,jasonatran/RIOT,miri64/RIOT,RIOT-OS/RIOT,OTAkeys/RIOT,kYc0o/RIOT,kaspar030/RIOT,authmillenon/RIOT,miri64/RIOT,OTAkeys/RIOT,RIOT-OS/RIOT,miri64/RIOT,authmillenon/RIOT,RIOT-OS/RIOT,authmillenon/RIOT,jasonatran/RIOT,kYc0o/RIOT,OTAkeys/RIOT,OlegHahm/RIOT,kYc0o/RIOT,authmillenon/RIOT,authmillenon/RIOT,RIOT-OS/RIOT,OlegHahm/RIOT,kYc0o/RIOT