Authn: use authenticator for grpc (#99573)

* Remove usage of grpc-authenticator

* Cleanup client construction code

pkg/storage/unified/sql/service.go

@@ -81,11 +81,7 @@ func ProvideUnifiedStorageGrpcService(
 
 	// FIXME: This is a temporary solution while we are migrating to the new authn interceptor
 	// grpcutils.NewGrpcAuthenticator should be used instead.
-	fallback := &grpc.Authenticator{Tracer: tracing}
-	authn, err := grpcutils.NewGrpcAuthenticatorWithFallback(cfg, reg, tracing, fallback)
-	if err != nil {
-		return nil, err
-	}
+	authn := grpcutils.NewAuthenticatorWithFallback(cfg, reg, tracing, &grpc.Authenticator{Tracer: tracing})
 
 	s := &service{
 		cfg:           cfg,