Remove SAMY and WHIRLPOOL, use double SHA256 instead

As discussed with @FellowTraveler.

Helpful comment for the reviewer:

```c++
// When calling SignContract or VerifySignature with "HASH256" as the hash type,
// the signature will use (hash256 . hash256) as a message digest.
// In this case, SignContractDefaultHash and VerifyContractDefaultHash are used,
// which resort to low level calls to accomplish non standard message digests.
// Otherwise, it will use whatever OpenSSL provides by that name (see
// GetOpenSSLDigestByName).
const String OTIdentifier::DefaultHashAlgorithm("HASH256");
```

The string here is also what you see on top of signed documents (was "SAMY" before).
This will abort code execution in this block such that the rest is an unreachable code path. What about replacing the whole if-block with

    OT_ASSERT(status);
?