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>
diff --git a/tests/test-string-output-visitor.c b/tests/test-string-output-visitor.c
index edff523..bfa3e8e 100644
--- a/tests/test-string-output-visitor.c
+++ b/tests/test-string-output-visitor.c
@@ -50,7 +50,7 @@
 static void visitor_output_teardown(TestOutputVisitorData *data,
                                     const void *unused)
 {
-    string_output_visitor_cleanup(data->sov);
+    visit_free(data->ov);
     data->sov = NULL;
     data->ov = NULL;
 }