Enable async backing on asset-hub-rococo (#2826)

The goal is to move all system chains on Rococo (+ other testnets?) to
use async backing.

Starting with `asset-hub-rococo` to get feedback, before I do the rest.

## Related

Example: https://github.com/paritytech/polkadot-sdk/pull/1619/
Guide:
https://github.com/w3f/polkadot-wiki/blob/master/docs/maintain/maintain-guides-async-backing.md

---------

Co-authored-by: Branislav Kontur <bkontur@gmail.com>
Co-authored-by: Dónal Murray <donal.murray@parity.io>

cumulus/parachains/runtimes/assets/asset-hub-westend/src/lib.rs

@@ -55,7 +55,6 @@ use frame_system::{
 use pallet_asset_conversion_tx_payment::AssetConversionAdapter;
 use pallet_nfts::{DestroyWitness, PalletFeatures};
 use pallet_xcm::EnsureXcm;
-pub use parachains_common as common;
 use parachains_common::{
 	impls::DealWithFees,
 	message_queue::*,