Remove pointer from the input of Digest nonce hashes (#549)

Remove pointer from the input of Digest nonce hashes (#549)

This is a follow-up to #491 (b20ce97), which hashed what was previously
revealed as plaintext. Removing the pointer from the input to the hash
removes the possibility that someone could recover a pointer by
reversing a hash. Having the pointer as input was not adding anything:
Squid remembers all outstanding nonces, so it really only requires
uniqueness, which is already guaranteed by the
authenticateDigestNonceFindNonce loop.