revert "qt: Use "fusion" style on macOS Big Sur with old Qt"

src/qt/bitcoin.cpp

@@ -51,7 +51,6 @@
 #include <QThread>
 #include <QTimer>
 #include <QTranslator>
-#include <QtGlobal>
 
 #if defined(QT_STATICPLUGIN)
 #include <QtPlugin>
@@ -467,13 +466,6 @@ int GuiMain(int argc, char* argv[])
     QCoreApplication::setAttribute(Qt::AA_EnableHighDpiScaling);
 #endif
 
-#if (QT_VERSION <= QT_VERSION_CHECK(5, 9, 8)) && defined(Q_OS_MACOS)
-    const auto os_name = QSysInfo::prettyProductName();
-    if (os_name.startsWith("macOS 11") || os_name.startsWith("macOS 10.16")) {
-        QApplication::setStyle("fusion");
-    }
-#endif
-
     BitcoinApplication app;
 
     /// 2. Parse command-line options. We do this after qt in order to show an error if there are problems parsing these