`polkadot-parachain`: Unify asset-hub authoring codepath with general AURA (#4932)

Recently thought about the special handling we have for asset-hub
chains. They started with relay chain consensus and transitioned to AURA
at some point. However, nobody should be authoring with relay chain
consensus on these chains anymore, the transition is long done.

I propose to remove this special handling, allowing us to unify one more
execution path.