Hei unateness is an attribute, for ex: when u give any input to an inverter its output will be inverted. It is said to having an negative unate. Similarly a buffer will rise when input is high and falls when changed to low. so it is called as positive unateness. whereas as dftrtl rightly pointed in a gate like xor if a value goes to 1, depending on other inputs it can either goes to 1 or 0. such a gate is defined with an non unate attribute. this is just an info and nothing to do with removing them.