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/paymentserver.h

@@ -118,7 +118,6 @@ protected:
 
 private:
     static bool readPaymentRequest(const QString& filename, PaymentRequestPlus& request);
-    std::string mapNetworkIdToName(CChainParams::Network networkId);
     bool processPaymentRequest(PaymentRequestPlus& request, SendCoinsRecipient& recipient);
     void fetchRequest(const QUrl& url);