- cleanup reference in remove method of array list

lists/arraylist/arraylist.go

@@ -82,6 +82,7 @@ func (list *List) Remove(index int) {
 		return
 	}
 
+	list.elements[index] = nil                                    // cleanup reference
 	copy(list.elements[index:], list.elements[index+1:list.size]) // shift to the left by one (slow operation, need ways to optimize this)
 	list.size -= 1