rpc: use existing rpc logger for wtclientrpc

The logger string used to identify the wtclient and wtclientrpc loggers
was the same, leading to being unable to modify the log level of the
wtclient logger as it would be overwritten with the wtclientrpc's one.
To simplify things, we decide to use the existing RPC logger for
wtclientrpc.

lnrpc/wtclientrpc/wtclient.go

@@ -115,8 +115,8 @@ func (c *WatchtowerClient) RegisterWithRootServer(grpcServer *grpc.Server) error
 	// all our methods are routed properly.
 	RegisterWatchtowerClientServer(grpcServer, c)
 
-	log.Debugf("WatchtowerClient RPC server successfully registered with " +
-		"root gRPC server")
+	c.cfg.Log.Debugf("WatchtowerClient RPC server successfully registered " +
+		"with  root gRPC server")
 
 	return nil
 }