Reuse val_native() in ExtractValue() and UpdateXML()

Currently the classes that implement SQL functions ExtractValue() and UpdateXML() use a special virtual Item method to transfer node sets values:

virtual String *val_raw(String*) { return 0; }

The top level Item returns 0, and the following classes override this method:

• Item_nodeset_func_rootelement
• Item_nodeset_func_union
• Item_nodeset_func_selfbyname
• Item_nodeset_func_childbyname
• Item_nodeset_func_descendantbyname
• Item_nodeset_func_ancestorbyname
• Item_nodeset_func_parentbyname
• Item_nodeset_func_attributebyname
• Item_nodeset_func_predicate
• Item_nodeset_func_elementbyindex

val_raw() is not used anywhere outside of the metioned XML functions implementation.

Let's remove Item::val_raw() and replace the XML functions relatec classes to use the generic method val_native().

This is needed to move XML functions into a function collection plugin later.