Remove the AIO transport as NIO is just faster

The AIO transport was added in the past as we hoped it would have better latency as the NIO transport. But in reality this was never the case.
So there is no reason to use the AIO transport at all. It just put more burden on us as we need to also support it and fix bugs.
Because of this we dedicided to remove it for now. It will stay in the master_with_aio_transport branch so we can pick it up later again if it is ever needed.