Remove duplicate code in listAddNodeTail (#11733)

Remove duplicate code that removes a node from the tail of a list.

src/adlist.c

@@ -126,16 +126,7 @@ list *listAddNodeTail(list *list, void *value)
     if ((node = zmalloc(sizeof(*node))) == NULL)
         return NULL;
     node->value = value;
-    if (list->len == 0) {
+    listLinkNodeTail(list, node);
-        list->head = list->tail = node;
-        node->prev = node->next = NULL;
-    } else {
-        node->prev = list->tail;
-        node->next = NULL;
-        list->tail->next = node;
-        list->tail = node;
-    }
-    list->len++;
     return list;
 }