Revert changes to evdns_nameserver_free

The bug that Azat was seeing wasn't from calling event_del() on a
non-added event: it was from calling event_del() on an uninitialized
event. But now that the event is always initialized with
event_assign(), it's no longer necessary to make the event_del()
conditional.
Tweak log messages and documentation for EVDNS_BASE_DISABLE_WHEN_INACTIVE