Darn, you have me confused. In your original post, you said that you don't even need the class. Later you said that you need to take both I & II.
Did you really 'bomb' it, as in failed the test, or did you just not perform up to your own hopes, as in getting a B or C? I'm not sure exactly what the story is, but if all you are worried about is having your final GPA be a 3.9__ as as opposed to a 4.0, there is nothing to worry about. It would be better have the calculus under your belt w/the 3.9_ than to NOT have it and have the 4.0. The only people you can impress with that tiny difference are yourself and, maybe, your parents. No one else, not even adcomms, are going to view that as a negative.
If your are REALLY bombing it, as in feeling totally lost and certain you'd fail, then withdrawing makes sense. In that case, it would also make sense to enroll in a lower level math course, too.
You're a junior and a chm major. Have you already taken chemistry and physics? If you have, then you HAVE been taking math because those subjects are really just math 'in the real world', so to speak.
If all you are worried about is getting a B, then lighten up on yourself. Having a B a your records can really take some pressure off. Suddenly, you no longer have the stress of trying to stay 'perfect'. Once you relax, who knows, you might wind up getting an A, in the end. Then you could laugh about how you freaked out over minor things at the beginning of the semester.
