qt, wallet, refactor: Drop unused `WalletModel::PaymentRequestExpired`

Also dead code has been removed.

src/qt/sendcoinsdialog.cpp

@@ -757,10 +757,6 @@ void SendCoinsDialog::processSendCoinsReturn(const WalletModel::SendCoinsReturn
     case WalletModel::AbsurdFee:
         msgParams.first = tr("A fee higher than %1 is considered an absurdly high fee.").arg(BitcoinUnits::formatWithUnit(model->getOptionsModel()->getDisplayUnit(), model->wallet().getDefaultMaxTxFee()));
         break;
-    case WalletModel::PaymentRequestExpired:
-        msgParams.first = tr("Payment request expired.");
-        msgParams.second = CClientUIInterface::MSG_ERROR;
-        break;
     // included to prevent a compiler warning.
     case WalletModel::OK:
     default:

src/qt/walletmodel.h

@@ -66,8 +66,7 @@ public:
         AmountWithFeeExceedsBalance,
         DuplicateAddress,
         TransactionCreationFailed, // Error returned when wallet is still locked
-        AbsurdFee,
-        PaymentRequestExpired
+        AbsurdFee
     };
 
     enum EncryptionStatus