sys/xtimer: clean up xtimer_mutex_lock_timeout
sys/xtimer: clean up xtimer_mutex_lock_timeout

Use `mutex_lock_cancelable()` and `mutex_cancel()` to implement
`xtimer_mutex_lock_timeout()`.

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