Trickle: remove vtimers before (re)scheduling it

Otherwise it might cause an endless loop in `queue.c` due to multiple nodes of the same instance (e.g. `dao_timer`).

(Would look like:
```
$5 = (struct queue_node_t *) 0x809b1e0 <dao_timer>
(gdb) print node->next->next->next->next
$6 = (struct queue_node_t *) 0x8083dfc <con_buf+7644>
(gdb) print node->next->next->next->next->next
$7 = (struct queue_node_t *) 0x809b1e0 <dao_timer>
(gdb) print node->next->next->next->next->next->next
$8 = (struct queue_node_t *) 0x8083dfc <con_buf+7644>
(gdb) print node->next->next->next->next->next->next->next
$9 = (struct queue_node_t *) 0x809b1e0 <dao_timer>
(gdb) print node->next->next->next->next->next->next->next->next
$10 = (struct queue_node_t *) 0x8083dfc <con_buf+7644>
(gdb) print node->next->next->next->next->next->next->next->next->next
$11 = (struct queue_node_t *) 0x809b1e0 <dao_timer>
```
)