cpu/samd5x: remove RTC workaround
cpu/samd5x: remove RTC workaround

This is no longer needed.

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