I have an RC stating that I wanted extra layers of plastic on the books to prevent damage from the weather. I initially put that there because of the hard snow we had during the winter months but now that my area is drying up, I don' t need that RC anymore.

I forgot to take that RC off and a book I wanted was accepted already. Would it be a good idea to PM the requester saying to ignore the RC about the extra layer so that they can save it for someone else? Or would it be better to just forget about it knowing it's already off my RC and the next person doesn't have to wrap a book in extra layers and not bug this requester with a chance of confusion.

Since the sender has already accepted the request, then they are okay with the RC as it was and I'd just leave it alone for this one.  You've removed it for the next one.  You could always thank them for the extra plastic when you receive the book.

I wouldn't PM them about it. All my books get wrapped in plastic when I send them out. That's unless someone has an RC to not wrap the books in plastic.