Get rid of PendingWalletTx class.

No reason for this class to exist if it doesn't have any code to run in the
destructor. e10e1e8db0 from
https://github.com/bitcoin/bitcoin/pull/16208 recently removed code destructor
code that would return an unused key if the transaction wasn't committed.

src/qt/sendcoinsdialog.cpp

@@ -392,7 +392,7 @@ void SendCoinsDialog::on_sendButton_clicked()
         accept();
         CoinControlDialog::coinControl()->UnSelectAll();
         coinControlUpdateLabels();
-        Q_EMIT coinsSent(currentTransaction.getWtx()->get().GetHash());
+        Q_EMIT coinsSent(currentTransaction.getWtx()->GetHash());
     }
     fNewRecipientAllowed = true;
 }