string-output-visitor: Favor new visit_free() function

Now that we have a polymorphic visit_free(), we no longer need
string_output_visitor_cleanup(); however, we still need to
expose the subtype for string_output_get_string().

Signed-off-by: Eric Blake <eblake@redhat.com>
Message-Id: <1465490926-28625-9-git-send-email-eblake@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>
