Don't cleanup doc reference of unlinked node.

This is cause if incorrect free in xmlFreeNode (DICT_FREE(cur->name)).

Author: ZigzagAK

xmlua/libxml2.lua

@@ -572,7 +572,6 @@ end
 
 function libxml2.xmlUnlinkNode(node)
   xml2.xmlUnlinkNode(node)
-  xml2.xmlSetTreeDoc(node, ffi.NULL)
   return ffi.gc(node, xml2.xmlFreeNode)
 end