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

This reverts commit 4e1154dfd1.

After bumping Qt version in depends to 5.12.10 in bitcoin/bitcoin#21376,
there are no reasons to use the Fusion style on macOS.

src/qt/bitcoin.cpp

@@ -52,7 +52,6 @@
 #include <QThread>
 #include <QTimer>
 #include <QTranslator>
-#include <QtGlobal>
 
 #if defined(QT_STATICPLUGIN)
 #include <QtPlugin>
@@ -467,13 +466,6 @@ int GuiMain(int argc, char* argv[])
     QApplication::setAttribute(Qt::AA_UseHighDpiPixmaps);
     QCoreApplication::setAttribute(Qt::AA_EnableHighDpiScaling);
 
-#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;
     QFontDatabase::addApplicationFont(":/fonts/monospace");