cdml:delete-nodes should only remove root nodes

The following example shouldn't remove the root node from the collection.

>>> variable $n;
>>>
>>> cddl:create(xs:QName("foo:coll"));
>>>
>>> $n := cdml:apply-insert-nodes(xs:QName("foo:coll"), <a><b/></a>);
>>>
>>> fn:trace(fn:count(cdml:collection(xs:QName("foo:coll"))), "#nodes after insert");
>>>
>>> cdml:delete-nodes($n/b);
>>>
>>> fn:trace(fn:count(cdml:collection(xs:QName("foo:coll"))), "#nodes after delete");