client/network: Remove default Kademlia DHT in favor of per protocol DHT (#5993)

Substrate was previously running both a chain specific Kademlia DHT as
well as the `/ipfs/kad/1.0.0` Kademlia DHT for backwards compatibility.
(See 4db45a85 for details.)

With the end of the transition period this commit removes the former
only leaving the latter and thus preventing the discovery of
incompatible nodes via Kademlia.