sys/xtimer: remove duplicat _xtimer_set declaration

sys/include/xtimer/implementation.h

@@ -63,14 +63,13 @@ static inline uint32_t _xtimer_lltimer_mask(uint32_t val)
  */
 uint64_t _xtimer_now64(void);
 int _xtimer_set_absolute(xtimer_t *timer, uint32_t target);
-void _xtimer_set64(xtimer_t *timer, uint32_t offset, uint32_t long_offset);
 void _xtimer_set(xtimer_t *timer, uint32_t offset);
+void _xtimer_set64(xtimer_t *timer, uint32_t offset, uint32_t long_offset);
 void _xtimer_periodic_wakeup(uint32_t *last_wakeup, uint32_t period);
 void _xtimer_set_msg(xtimer_t *timer, uint32_t offset, msg_t *msg, kernel_pid_t target_pid);
 void _xtimer_set_msg64(xtimer_t *timer, uint64_t offset, msg_t *msg, kernel_pid_t target_pid);
 void _xtimer_set_wakeup(xtimer_t *timer, uint32_t offset, kernel_pid_t pid);
 void _xtimer_set_wakeup64(xtimer_t *timer, uint64_t offset, kernel_pid_t pid);
-void _xtimer_set(xtimer_t *timer, uint32_t offset);
 int _xtimer_msg_receive_timeout(msg_t *msg, uint32_t ticks);
 int _xtimer_msg_receive_timeout64(msg_t *msg, uint64_t ticks);