libpod/events: refactor to eliminate unused code

Shuffle the code around to eliminate "unused" warnings when linting
with various GOOS and build tags.

The only change in functionality should be that now NewEventer
returns ErrNoJournaldLogging (rather than "unknown event logger type")
on freebsd when journald is requested.

Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>

libpod/events/nullout.go

@@ -19,12 +19,6 @@ func (e EventToNull) Read(ctx context.Context, options ReadOptions) error {
 	return errors.New("cannot read events with the \"none\" backend")
 }
 
-// newNullEventer returns a new null eventer.  You should only do this for
-// the purposes of internal libpod testing.
-func newNullEventer() Eventer {
-	return EventToNull{}
-}
-
 // String returns a string representation of the logger
 func (e EventToNull) String() string {
 	return "none"