sync: Fix incorrect "nothing to transfer" message using --delete-before

Before this change the first pass of --delete-before would output
"There was nothing to transfer" and then proceed to transfer things.

This makes sure the message isn't printed in the delete phase.

See: https://forum.rclone.org/t/incorrect-debug-output/15267

fs/sync/sync.go

@@ -844,7 +844,7 @@ func (s *syncCopyMove) run() error {
 	// Read the error out of the context if there is one
 	s.processError(s.ctx.Err())
 
-	if accounting.Stats(s.ctx).GetTransfers() == 0 {
+	if s.deleteMode != fs.DeleteModeOnly && accounting.Stats(s.ctx).GetTransfers() == 0 {
 		fs.Infof(nil, "There was nothing to transfer")
 	}