gui: remove unreachable AmountWithFeeExceedsBalance error

Since #20640, the 'createTransaction' does no longer retrieve the
fee if the process fails due to insufficient funds.

But, since #25269, 'createTransaction' retrieves an error message
indicating that the total transaction amount exceeds the wallet
available balance when fees are included.

So this enum is no longer needed.

Author: furszy

src/qt/walletmodel.cpp

@@ -214,12 +214,7 @@ WalletModel::SendCoinsReturn WalletModel::prepareTransaction(WalletModelTransact
         if (fSubtractFeeFromAmount && newTx)
             transaction.reassignAmounts(nChangePosRet);
 
-        if(!newTx)
-        {
-            if(!fSubtractFeeFromAmount && (total + nFeeRequired) > nBalance)
-            {
-                return SendCoinsReturn(AmountWithFeeExceedsBalance);
-            }
+        if (!newTx) {
             Q_EMIT message(tr("Send Coins"), QString::fromStdString(util::ErrorString(res).translated),
                 CClientUIInterface::MSG_ERROR);
             return TransactionCreationFailed;