tests/driver_at86rf2xx_aes: remove unnessesary xtimer.h import
tests/driver_at86rf2xx_aes: remove unnessesary xtimer.h import

ant9000/RIOT,RIOT-OS/RIOT,RIOT-OS/RIOT,ant9000/RIOT,ant9000/RIOT,kaspar030/RIOT,RIOT-OS/RIOT,RIOT-OS/RIOT,RIOT-OS/RIOT,ant9000/RIOT,ant9000/RIOT,kaspar030/RIOT,kaspar030/RIOT,kaspar030/RIOT,kaspar030/RIOT