Drop the BIP39 passphrase from cbk

The seed is always computed with an empty passphrase; Params is just
the mnemonic and the BIP85 index.

README.md

@@ -66,9 +66,9 @@ func main() {
         panic(err)
     }
 
-    // The same mnemonic (and optional passphrase) always yields the same
-    // keypair, on any machine. The keys are plain crypto_box keys for use
-    // with golang.org/x/crypto/nacl/box.
+    // The same mnemonic always yields the same keypair, on any machine.
+    // The keys are plain crypto_box keys for use with
+    // golang.org/x/crypto/nacl/box.
     keypair, err := cbk.New(cbk.Params{Mnemonic: mnemonic})
     if err != nil {
         panic(err)
@@ -138,8 +138,8 @@ independent keypairs:
 1. The mnemonic is validated and canonicalized with
    `bip39wordlist.NormalizeWordlist`.
 2. The standard 64-byte BIP39 seed is computed with PBKDF2-HMAC-SHA512 (2048
-   rounds, salt `"mnemonic" + passphrase`), exactly as a BIP39 wallet would, so
-   the optional passphrase works as the usual "25th word".
+   rounds, salt `"mnemonic"`), exactly as a BIP39 wallet would with an empty
+   passphrase. BIP39 passphrases are deliberately not supported.
 3. The seed becomes a BIP32 master key.
 4. BIP85's HEX application derives 32 bytes of entropy at
    `m/83696968'/128169'/32'/{index}'`; those bytes are the private key.
@@ -148,8 +148,8 @@ independent keypairs:
 The resulting keys are plain 32-byte crypto_box keys for use with
 `golang.org/x/crypto/nacl/box`. The API:
 
-- `cbk.New(cbk.Params{Mnemonic, Passphrase, Index})` and `cbk.FromMnemonic(m)`
-  (no passphrase, index 0) return a `CryptoBoxKeypair`.
+- `cbk.New(cbk.Params{Mnemonic, Index})` and `cbk.FromMnemonic(m)` (index 0)
+  return a `CryptoBoxKeypair`.
 - `CryptoBoxKey.Hex()`, `CryptoBoxKey.Bytes()`, and the `PublicKeyHex`,
   `PrivateKeyHex`, `PublicKeyBytes`, `PrivateKeyBytes` accessors on the keypair.