Correct small bugs in the v-for array refs examples

[skirtles-code]

I've fixed 3 problems with these examples.

1. ref is unused, close ref is unused in array example #766.
2. The setItemRef function needs an if check, otherwise removed nodes will be pushed as null. This is effectively the same problem I reported in Composition API template refs with v-for example - update problem #353, though in a different part of the docs.
3. The setup function should not return itemRefs, it's highly misleading. itemRefs is reassigned in onBeforeUpdate but that won't update the value returned from setup. There are other ways to fix this (set the length to 0 or wrap it in a ref) but this seemed the simplest.