lib, lib-master: Remove event.call_free

lib, lib-master: Remove event.call_free

Just call the EVENT_CALLBACK_TYPE_FREE always. Having this flag isn't
really going to affect the performance, and it might be useful in the
future to work this way.

Only the unit test was relying on call_free to be different from
id_sent_to_stats. This commit changes the unit test to check for the
END strings where necessary.