[Qt] Use identical strings for expired payment request message

- used in sendcoinsdialog.cpp and paymentserver.cpp
- removes an unneded translation string

Author: Philip Kaufmann

src/qt/sendcoinsdialog.cpp

@@ -531,7 +531,7 @@ void SendCoinsDialog::processSendCoinsReturn(const WalletModel::SendCoinsReturn
         msgParams.first = tr("A fee higher than %1 is considered an absurdly high fee.").arg(BitcoinUnits::formatWithUnit(model->getOptionsModel()->getDisplayUnit(), 10000000));
         break;
     case WalletModel::PaymentRequestExpired:
-        msgParams.first = tr("Payment request expired!");
+        msgParams.first = tr("Payment request expired.");
         msgParams.second = CClientUIInterface::MSG_ERROR;
         break;
     // included to prevent a compiler warning.