remove support for filter in the form of Unist node
remove support for filter in the form of Unist node

When a single node is mentioned as a possible filter value, a request
for multiple nodes follows, which is neither part of current
`unist-util-is` API nor frankly I think should it be, so at this point
I would have to reimplement (again) the `array.indexOf(node) >= 0` test,
but I don't want to repeat myself. So yeah, no node tests :smile:

eush77/unist-util-index