op-node: ComputeL2OutputRoot cleanup

Cleans up the implementation of the `ComputeL2OutputRoot`
function. No need for the intermediate `bytes.Buffer`.

op-node/rollup/output_root.go

 package rollup
 
 import (
-	"bytes"
-
 	"github.com/ethereum-optimism/optimism/op-node/eth"
 	"github.com/ethereum/go-ethereum/common"
 	"github.com/ethereum/go-ethereum/crypto"
 )
 
 func ComputeL2OutputRoot(l2OutputRootVersion eth.Bytes32, blockHash common.Hash, blockRoot common.Hash, storageRoot common.Hash) eth.Bytes32 {
-	var buf bytes.Buffer
-	buf.Write(l2OutputRootVersion[:])
-	buf.Write(blockRoot.Bytes())
-	buf.Write(storageRoot[:])
-	buf.Write(blockHash.Bytes())
-	return eth.Bytes32(crypto.Keccak256Hash(buf.Bytes()))
+	digest := crypto.Keccak256Hash(
+		l2OutputRootVersion[:],
+		blockRoot.Bytes(),
+		storageRoot[:],
+		blockHash.Bytes(),
+	)
+	return eth.Bytes32(digest)
 }