Migrate validator client to clap derive (#6300)

Partially #5900


  Migrate the validator client cli to clap derive

beacon_node/src/cli.rs

@@ -18,15 +18,6 @@ pub fn cli_app() -> Command {
         /*
          * Configuration directory locations.
          */
-        .arg(
-            Arg::new("help")
-            .long("help")
-            .short('h')
-            .help("Prints help information")
-            .action(ArgAction::HelpLong)
-            .display_order(0)
-            .help_heading(FLAG_HEADER)
-        )
         .arg(
             Arg::new("network-dir")
                 .long("network-dir")