factor: remove unused method "zero"

src/uu/factor/src/numeric/montgomery.rs

@@ -43,12 +43,10 @@ pub(crate) trait Arithmetic: Copy + Sized {
     fn one(&self) -> Self::ModInt {
         self.to_mod(1)
     }
+
     fn minus_one(&self) -> Self::ModInt {
         self.to_mod(self.modulus() - 1)
     }
-    fn zero(&self) -> Self::ModInt {
-        self.to_mod(0)
-    }
 }
 
 #[derive(Clone, Copy, Debug)]