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Unused and unpublished.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/ed448.rs

@@ -66,12 +66,6 @@ impl Ciphersuite for Ed448 {
     Point::generator()
   }
 
-  fn reduce_512(mut scalar: [u8; 64]) -> Self::F {
-    let res = Self::hash_to_F(b"Ciphersuite-reduce_512", &scalar);
-    scalar.zeroize();
-    res
-  }
-
   fn hash_to_F(dst: &[u8], data: &[u8]) -> Self::F {
     Scalar::wide_reduce(Self::H::digest([dst, data].concat()).as_ref().try_into().unwrap())
   }