Remove TODO from extract_p

The protocol spec now returns \mathbb{P}_x instead of a bit sequence,
matching what we do here.

src/spec.rs

@@ -101,8 +101,6 @@ pub(crate) fn ka_orchard(sk: &pallas::Scalar, b: &pallas::Point) -> pallas::Poin
 ///
 /// [concreteextractorpallas]: https://zips.z.cash/protocol/nu5.pdf#concreteextractorpallas
 pub(crate) fn extract_p(point: &pallas::Point) -> pallas::Base {
-    // TODO: Should we return the actual bits in a Vec, or allow the caller to use
-    // PrimeField::to_le_bits on the returned pallas::Base?
     if let Some((x, _)) = point.to_affine().get_xy().into() {
         x
     } else {