De-duplicate `enable_light_client_server`

[michaelsproul]

The enable_light_client_server config is parsed 3 separate times at startup, and shimmed into 3 different structs. It would be nice if we could just define it in one place and have a single source of truth.

lighthouse/beacon_node/src/config.rs

Lines 178 to 179 in 0344f68

	client_config.http_api.enable_light_client_server =
	cli_args.get_flag("light-client-server");

lighthouse/beacon_node/src/config.rs

Lines 182 to 184 in 0344f68

	if cli_args.get_flag("light-client-server") {
	client_config.chain.enable_light_client_server = true;
	}

lighthouse/beacon_node/src/config.rs

Lines 1421 to 1422 in 0344f68

	// Light client server config.
	config.enable_light_client_server = parse_flag(cli_args, "light-client-server");

We are flipping the flag polarity in this PR, but the same issue remains:

• Enable Light Client server by default #6950