message: network-grpc: Remove a disused protobuf type
  BlockSubscriptionRequest is no more.
diff: @@ -49,9 +49,6 @@ message Transaction {
  bytes content = 1;
  }
  -// Request message for method SubscribeToBlocks.
  -message BlockSubscriptionRequest {}
  -
  // Gossip message.
  message GossipMessage {
  message NodeId {
  