chore: Drop the deprecated Cluster Client

[He-Pin]

Motivation: The Cluster client is deprecated since Akka 2.6.0 and suggests using Pekko Grpc instead.

Modification:

Delete Cluster client code and doc.

Result: Clean code.

[pjfanning]

I'm not 100% in favour of this. pekko-grpc is pretty complicated to use - this class is somewhat easier to use.

[He-Pin]

But seems this will not provide more function than it is now, but with grpc, a user can easy to write an Cluster endpoint.

[pjfanning]

Unless we can document how users are meant to use pekko-grpc in detail, I would favour not deleting this code - even possibly undeprecating it.

[He-Pin]

Just a gRPC server which receive the message and then forwarding to the cluster:)

[pjfanning]

Just a gRPC server which receive the message and then forwarding to the cluster:)

That is much more complicated than us providing a ClusterClient.