lib: Remove NodeInterface

I'll create an iterator for nodes and drop that every node is a SplDoublyLinkedList.

refs #6630

library/Icinga/Data/Tree/NodeInterface.php

@@ -1,25 +0,0 @@
-<?php
-/* Icinga Web 2 | (c) 2013-2015 Icinga Development Team | GPLv2+ */
-
-namespace Icinga\Data\Tree;
-
-use RecursiveIterator;
-
-interface NodeInterface extends RecursiveIterator
-{
-    /**
-     * Create a new node from the given value and insert the node as the last child of this node
-     *
-     * @param   mixed           $value  The node's value
-     *
-     * @return  NodeInterface           The appended node
-     */
-    public function appendChild($value);
-
-    /**
-     * Get the node's value
-     *
-     * @return mixed
-     */
-    public function getValue();
-}