Remove redundant impl::tree_impl::okay_ field
Remove redundant impl::tree_impl::okay_ field

No real changes, just remove the field which was only set to false when when
any errors were logged, making it unnecessary: we can just use
messages_.has_errors() instead of it.

vslavik/xmlwrapp,vadz/xmlwrapp,vadz/xmlwrapp,vslavik/xmlwrapp,vadz/xmlwrapp,vslavik/xmlwrapp