Remove the poorly-designed reduce_512 API

Unused and unpublished. This was only added in the FCMP++ branch as a quick fix
for performance reasons. Finding a better API is still a tricky question, but
this API is _bad_.

crypto/ciphersuite/src/dalek.rs

@@ -28,12 +28,6 @@ macro_rules! dalek_curve {
         $Point::generator()
       }
 
-      fn reduce_512(mut scalar: [u8; 64]) -> Self::F {
-        let res = Scalar::from_bytes_mod_order_wide(&scalar);
-        scalar.zeroize();
-        res
-      }
-
       fn hash_to_F(dst: &[u8], data: &[u8]) -> Self::F {
         Scalar::from_hash(Sha512::new_with_prefix(&[dst, data].concat()))
       }