How to tactfully request removal of my name from co-authorship?
You have a third choice:

Do not forget that all the authors should sign a contract before publishing a paper.
I am not telling you not to sign it.
However, it would be a good approach if you told your professor


  I am sure that huge improvements can still be made on this paper.
  Could you please delay the submission and let me work on the paper for
  another period of time?


This way, you told that paper is not of publication quality and implicated that you're not comfortable with your name on it.
If the professor still insists on publishing, only then you may say


  It's not like I'm not trusting your judgmental skills but when I know
  that I can do better, I cannot do with less. So please do not
  misunderstand me but I don't want to be a coauthor of a work that is
  less than my potential.


If your professor is professional enough, he will hear what you say.

Maybe not the best solution, but I would go with this one.