trickle: remove msg_time member from struct

The msg_time member is only locally used and set before read. It can
be removed as the state kept by the member is not used.

sys/include/trickle.h

@@ -58,7 +58,6 @@ typedef struct {
     trickle_callback_t callback;    /**< callback function and parameter that
                                          trickle calls after each interval */
     msg_t msg;                      /**< the msg_t to use for intervals */
-    uint64_t msg_time;              /**< interval in ms */
     xtimer_t msg_timer;             /**< xtimer to send a msg_t to the target
                                          thread for a new interval */
 } trickle_t;

sys/trickle/trickle.c

@@ -51,8 +51,8 @@ void trickle_interval(trickle_t *trickle)
     /* old_interval == trickle->I / 2 */
     trickle->t = random_uint32_range(old_interval, trickle->I);
 
-    trickle->msg_time = (trickle->t + diff) * MS_PER_SEC;
-    xtimer_set_msg64(&trickle->msg_timer, trickle->msg_time, &trickle->msg,
+    uint64_t msg_time = (trickle->t + diff) * US_PER_MS;
+    xtimer_set_msg64(&trickle->msg_timer, msg_time, &trickle->msg,
                      trickle->pid);
 }