dropbox: fix hang on quit with --dropbox-batch-mode off

This problem was created by the fact that we are much more diligent
about calling Shutdown now, and the dropbox backend had a hang if the
batch mode was "off" in the Shutdown method.

See: https://forum.rclone.org/t/dropbox-lsjson-in-1-59-stuck-on-commiting-upload/31853

backend/dropbox/batcher.go

@@ -304,6 +304,9 @@ outer:
 //
 // Can be called from atexit handler
 func (b *batcher) Shutdown() {
+	if !b.Batching() {
+		return
+	}
 	b.shutOnce.Do(func() {
 		atexit.Unregister(b.atexit)
 		fs.Infof(b.f, "Commiting uploads - please wait...")