Parachains-Aura: Only produce once per slot (#3308)

Given how the block production is driven for Parachains right now, with
the enabling of async backing we would produce two blocks per slot.
Until we have a proper collator implementation, the "hack" is to prevent
the production of multiple blocks per slot.


Closes: https://github.com/paritytech/polkadot-sdk/issues/3282