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 (sha256 . sha256) 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).