gui: Show watchonly balance only for Legacy wallets

Descriptor wallets do not have a watchonly balance as wallets are
designated watchonly or not. Thus we should not be displaying the empty
watchonly balance for descriptor wallets.

Author: achow101

src/qt/sendcoinsdialog.cpp

@@ -703,7 +703,7 @@ void SendCoinsDialog::setBalance(const interfaces::WalletBalances& balances)
         CAmount balance = balances.balance;
         if (model->wallet().hasExternalSigner()) {
             ui->labelBalanceName->setText(tr("External balance:"));
-        } else if (model->wallet().privateKeysDisabled()) {
+        } else if (model->wallet().isLegacy() && model->wallet().privateKeysDisabled()) {
             balance = balances.watch_only_balance;
             ui->labelBalanceName->setText(tr("Watch-only balance:"));
         }