kqnotify: drop EV_CLEAR (edge triggering)

kqnotify: drop EV_CLEAR (edge triggering)

I'm not entirely certain how it works with the way we use
kevent.  I do know IO::KQueue has hard-coded kevent retrievals
to 1000 events so it's conceivable we'd end up missing wakeups
as we don't loop or requeue in callers.  So just rely on the
*BSD kernel to provided requeue behavior for us by using
level-triggering.

In any case, this seems to workaround t/dir_idle.t failures
on Dragonfly due to a tmpfs bug in all versions up to v6.4.