add NetworkIDString() to chainparams

- returns the BIP70 network string
- use that new function in the core and GUI code and remove unused code
  and functions

src/qt/rpcconsole.cpp

@@ -316,7 +316,7 @@ void RPCConsole::setClientModel(ClientModel *model)
         ui->buildDate->setText(model->formatBuildDate());
         ui->startupTime->setText(model->formatClientStartupTime());
 
-        ui->networkName->setText(model->getNetworkName());
+        ui->networkName->setText(QString::fromStdString(Params().NetworkIDString()));
     }
 }