ecdh: remove use of hashes in example code and tests

We have a curious test where we confirm that we can match upstream's code
with direct calls to sha256. This is nice, but completely unnecessary as
a unit test.

Secondly, we have example code where we demonstrate how to use bitcoin_hashes
to write a custom ECDH nonce function. This is valuable, but not reason by
itself to have an extra dependency, so we stick `ignore` on it to prevent
the Rust compiler from attempting to compile it.

Author: apoelstra

src/ecdh.rs

@@ -110,10 +110,9 @@ impl AsRef<[u8]> for SharedSecret {
 /// 64 bytes representing the (x,y) co-ordinates of a point on the curve (32 bytes each).
 ///
 /// # Examples
-/// ```
-/// # #[cfg(all(feature = "hashes", feature = "rand", feature = "std"))] {
-/// # use secp256k1::{ecdh, rand, Secp256k1, PublicKey, SecretKey};
-/// # use secp256k1::hashes::{Hash, sha512};
+/// ```ignore
+/// use bitcoin_hashes::{Hash, sha512};
+/// use secp256k1::{ecdh, rand, Secp256k1, PublicKey, SecretKey};
 ///
 /// let s = Secp256k1::new();
 /// let (sk1, pk1) = s.generate_keypair(&mut rand::rng());
@@ -124,7 +123,6 @@ impl AsRef<[u8]> for SharedSecret {
 /// let point2 = ecdh::shared_secret_point(&pk1, &sk2);
 /// let secret2 = sha512::Hash::hash(&point2);
 /// assert_eq!(secret1, secret2)
-/// # }
 /// ```
 pub fn shared_secret_point(point: &PublicKey, scalar: &SecretKey) -> [u8; 64] {
     let mut xy = [0u8; 64];
@@ -224,32 +222,6 @@ mod tests {
         assert_eq!(y, new_y);
     }
 
-    #[test]
-    #[cfg(not(secp256k1_fuzz))]
-    #[cfg(all(feature = "hashes", feature = "rand", feature = "std"))]
-    fn hashes_and_sys_generate_same_secret() {
-        use hashes::{sha256, Hash, HashEngine};
-
-        use crate::ecdh::shared_secret_point;
-
-        let s = Secp256k1::signing_only();
-        let (sk1, _) = s.generate_keypair(&mut rand::rng());
-        let (_, pk2) = s.generate_keypair(&mut rand::rng());
-
-        let secret_sys = SharedSecret::new(&pk2, &sk1);
-
-        let xy = shared_secret_point(&pk2, &sk1);
-
-        // Mimics logic in `bitcoin-core/secp256k1/src/module/main_impl.h`
-        let version = (xy[63] & 0x01) | 0x02;
-        let mut engine = sha256::HashEngine::default();
-        engine.input(&[version]);
-        engine.input(&xy.as_ref()[..32]);
-        let secret_bh = sha256::Hash::from_engine(engine);
-
-        assert_eq!(secret_bh.as_byte_array(), secret_sys.as_ref());
-    }
-
     #[test]
     #[cfg(all(feature = "serde", feature = "alloc"))]
     fn serde() {