chore(evm-node): remove stale chain-spec FIXME

`crates/evm-node/src/node.rs` carries:

    // FIXME use the ethereum chain spec temporary, we need to define Arc chain spec
    // original traits for ChainSpec in this file `Hardforks + EthereumHardforks + EthExecutorSpec`

Arc's chain spec already exists and this file already uses it: `ArcChainSpec`
is defined in `crates/execution-config/src/chainspec.rs` and node.rs binds
`ChainSpec = ArcChainSpec` throughout. That was already true in the first
public commit (a1235c0), where node.rs used `ArcChainSpec` 10 times — so the
FIXME has been stale since it landed.

The second line documented the upstream reth file's ChainSpec bounds and only
made sense as a continuation of the FIXME, so it goes too.

Comments only; no behaviour change.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

Author: mehmetkr-31

crates/evm-node/src/node.rs

@@ -79,9 +79,6 @@ use arc_execution_config::chainspec::ArcChainSpec;
 use arc_execution_payload::payload::ArcNetworkPayloadBuilderBuilder;
 use arc_execution_txpool::{ArcPoolBuilder, InvalidTxList, InvalidTxListConfig};
 
-// FIXME use the ethereum chain spec temporary, we need to define Arc chain spec
-// original traits for ChainSpec in this file `Hardforks + EthereumHardforks + EthExecutorSpec`
-
 use crate::rpc_middleware::{
     ArcRpcLayer, ARC_RPC_MAX_BATCH_ENTRIES_DEFAULT, DEFAULT_TX_RELAY_TIMEOUT,
 };