Even if Hitler somehow decided he'd rather not do one of his main goals of exterminating the Jews, sending them all to Madagascar would be far too impractical. You can't just send thousands upon thousands of people to another nation without a lot of effort, resources, and international scourge, and Hitler can't afford the latter depending on the time without making the Allied nations lean ever closer towards declaring war on him. And this would not stop Hitler's need to destroy all the Slavs through war, and he won't divert resources to get rid of the Jews through less practical means if it's a deterrent to him.
I doubt many Jews would like being forced onto some island instead of their own homes, and I really doubt the French government will go along with this plan as well.
And if Hitler does this during the war, well, how will he get all these Jews to Africa when the Royal Navy is on his ass, and the Suez Canal is closed as well?