tests/thread_priority_inversion: remove unneeded xtimer_init()
tests/thread_priority_inversion: remove unneeded xtimer_init()

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