Bitcoin-Qt: remove obsolete modal flag from GUI APIs

- as we (can) supply the CClientUIInterface::MODAL flag via the style
  parameter, we don't need a separate bool for checking the modality

src/qt/walletmodel.h

@@ -148,7 +148,7 @@ signals:
     void requireUnlock();
 
     // Asynchronous message notification
-    void message(const QString &title, const QString &message, bool modal, unsigned int style);
+    void message(const QString &title, const QString &message, unsigned int style);
 
 public slots:
     /* Wallet status might have changed */