Simplify CryptoUtil helper

See MetacoSA/NBitcoin#969 (comment)

Author: knocte

src/GWallet.Backend/UtxoCoin/Lightning/ChannelManagement.fs

@@ -129,11 +129,7 @@ type ChannelInfo =
 
 module internal CryptoUtil =
     let internal AccountPrivateKeyToNodeMasterPrivKey (accountKey: Key): NodeMasterPrivKey =
-        let privateKeyBytesLength = 32
-        let bytes: array<byte> = Array.zeroCreate privateKeyBytesLength
-        use bytesStream = new MemoryStream(bytes)
-        let stream = NBitcoin.BitcoinStream(bytesStream, true)
-        accountKey.ReadWrite stream
+        let bytes = accountKey.ToBytes ()
         let hashed = NBitcoin.Crypto.Hashes.DoubleSHA256 bytes
         NodeMasterPrivKey <| NBitcoin.ExtKey (hashed.ToString())