If you successfully withdraw the I-130, the I-864 becomes void. But certainly, mention both forms in just one letter explaining that you wish to withdraw any and all petitions and forms relating to #your case number (you'll need that), along with the fact that you're separated and are filing for divorce.
And you're very welcome - that whole I-864 thing can financially ruin the sponsoring US citizen, and unfortunately many people don't really understand the ramifications when they sign it. Many sponsored immigrants have used the I-864 to obtain spousal support where ordinarily they wouldn't get a dime (for example, a short marriage would generally not result in spousal support).
Now when the baby is born, you'll need to file to establish paternity if you have any doubts - and if the baby is yours, you can of course file for some sort of custody, visitation etc., and child support.

