Merge bitcoin-core/gui#246: Revert "qt: Use "fusion" style on macOS Big Sur with old Qt"

77833a364a Revert "qt: Use "fusion" style on macOS Big Sur with old Qt" (Hennadii Stepanov)

Pull request description:

  This PR reverts workaround introduced in #177.

  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.

ACKs for top commit:
  leonardojobim:
    tACK https://github.com/bitcoin-core/gui/pull/246/commits/77833a364a09c463e088069faf4d32dc5cea7242. Tested on macOS Big Sur v11.2.3
  jarolrod:
    ACK 77833a364a
  Talkless:
    utACK 77833a364a

Tree-SHA512: f704f2027dd380dfc604231e3606a036a8be891aeeddf643c474131014fa080e123b42836ac643a2064fe7a5a018fa8b9aa61a31f9da1d15880de6a36c4c0d54

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");