Reincorporate deleting children nodes/descendant tree function

I was reviewing the latest dev (2009-Nov-1) and noticed that deleting a node's children was not functioning. It appears that the functionality behind it was removed, as well as a recursive function for getting the descendant tree. I added the code back in and created a patch. The code is based upon the 6.x-1.x branch and modified to use the new _nodehierarchy_get_children_menu_links function in 2.x.