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/clientmodel.h

@@ -56,8 +56,6 @@ public:
     double getVerificationProgress() const;
     QDateTime getLastBlockDate() const;
 
-    //! Return network (main, testnet3, regtest)
-    QString getNetworkName() const;
     //! Return true if core is doing initial block download
     bool inInitialBlockDownload() const;
     //! Return true if core is importing blocks