Remove non-async backing code from node

[alindima]

Now that async backing has been successfully enabled on all networks, we could remove a lot of code that dealt with async backing not being present (the AsyncBackingParams runtime API not being present). The runtime API is actually the trigger for the node subsystems (not the actual values of the params).
If sync backing is desired, one may still use zeroed AsyncBackingParams.

Some subsystems which explicitly dealt with this: collator-protocol, backing, provisioner.

I believe we may also remove the legacy statement-distribution subsystem