Remove an XXX comment about using something better than array_ref!().

It turns out array_ref!() is probably the best way, or, at least, we're
already using it everywhere.

src/scalar.rs

@@ -584,7 +584,6 @@ impl UnpackedScalar {
             do_carry_uncentered(&mut limbs, i);
         }
 
-        // XXX better way to get [i64;12] from [i64;24] ?
         UnpackedScalar(*array_ref!(limbs, 0, 12))
     }