In my opinion, 'it is' functions as an intensifier. Taken as a whole, this sentence is strong and 'it is' contributes to the strength of feeling in the sentence. It's written to be nearly an insult or definitely a dismissal of the subject at hand.

Yes, leaving the "it is" out is fine. Personally, I would say, "Whatever you're talking about is definitely untrue", or "Whatever it is you're talking about is definitely untrue" if you wanted to make it stronger like Jerry said.