chore: remove mekong network config (#7400)

The network is based on devnet-4 specs and it's no longer possible to
join it using our latest unstable since we merged devnet-5 changes.

For users that still wanna experiment on Mekong it's possible to use our
[1.25.0](https://github.com/ChainSafe/lodestar/releases/tag/v1.25.0)
release.

packages/cli/src/networks/index.ts

@@ -21,18 +21,16 @@ import * as ephemery from "./ephemery.js";
 import * as gnosis from "./gnosis.js";
 import * as holesky from "./holesky.js";
 import * as mainnet from "./mainnet.js";
-import * as mekong from "./mekong.js";
 import * as sepolia from "./sepolia.js";
 
-export type NetworkName = "mainnet" | "dev" | "gnosis" | "sepolia" | "holesky" | "chiado" | "ephemery" | "mekong";
+export type NetworkName = "mainnet" | "dev" | "gnosis" | "sepolia" | "holesky" | "chiado" | "ephemery";
 export const networkNames: NetworkName[] = [
   "mainnet",
   "gnosis",
   "sepolia",
   "holesky",
   "chiado",
   "ephemery",
-  "mekong",
 
   // Leave always as last network. The order matters for the --help printout
   "dev",
@@ -72,8 +70,6 @@ export function getNetworkData(network: NetworkName): {
       return chiado;
     case "ephemery":
       return ephemery;
-    case "mekong":
-      return mekong;
     default:
       throw Error(`Network not supported: ${network}`);
   }