Remove unused ThreadSafeAskFee from ui_interface

ThreadSafeAskFee is effectively unused. It is only called
when the fAskFee parameter on SendMoney or SendMoneyToDestination
is true, which never happens. Remove it.

src/qt/bitcoingui.h

@@ -135,16 +135,6 @@ public slots:
     */
     void message(const QString &title, const QString &message, unsigned int style, bool *ret = NULL);
 
-    /** Asks the user whether to pay the transaction fee or to cancel the transaction.
-       It is currently not possible to pass a return value to another thread through
-       BlockingQueuedConnection, so an indirected pointer is used.
-       https://bugreports.qt-project.org/browse/QTBUG-10440
-
-      @param[in] nFeeRequired       the required fee
-      @param[out] payFee            true to pay the fee, false to not pay the fee
-    */
-    void askFee(qint64 nFeeRequired, bool *payFee);
-
     bool handlePaymentRequest(const SendCoinsRecipient& recipient);
 
     /** Show incoming transaction notification for new transactions. */