Use the network backend from the parachain config

For consistency. For example `start_node()` uses
`FullNetworkConfiguration::<_, _, Net>::new(&parachain_config.network)`

cumulus/polkadot-parachain/src/command.rs

@@ -659,7 +659,7 @@ pub fn run() -> Result<()> {
  info!("Parachain Account: {}", parachain_account);
  info!("Is collating: {}", if config.role.is_authority() { "yes" } else { "no" });
 
- match polkadot_config.network.network_backend {
+ match config.network.network_backend {
  sc_network::config::NetworkBackendType::Libp2p =>
  start_node::<sc_network::NetworkWorker<_, _>>(
  config,