qt, refactor: Drop intermediate BitcoinApplication::shutdownResult slot

src/qt/bitcoin.h

@@ -73,7 +73,6 @@ public Q_SLOTS:
     void initializeResult(bool success, interfaces::BlockAndHeaderTipInfo tip_info);
     /// Request core shutdown
     void requestShutdown();
-    void shutdownResult();
     /// Handle runaway exceptions. Shows a message box with the problem and quits the program.
     void handleRunawayException(const QString &message);