An ultrarealistic penis prop can claim much credit for the success of the first ”Scary Movie” — and, saints be praised, in Scary Movie 2, it’s baaaaack. But its talents are wasted in a flaccid, no-groans-barred spoof of ”The Haunting,” itself a snoozer no one saw. The Wayans brothers trowel on the playground scatology, but many of the alleged ”gags” simply aren’t jokes in any traditional sense. (Musical cues help you time your guffaws.) A pity, because the penis really deserved a better flick.