@Kapow Unfortunately I don't think there is a way to remove/undo rating. Cursory search through Mangadex reddit and forums show no legitimate answers to this question and mods don't seem to answer either.
Maybe you could just read the series and change your rating later? Or give it a generic 6-7 if you don't plan to read it? I know it's not the best way to go about this but it's pretty much the only way I can think of, and that I would do if I accidentally rated an unread series too.