App/main.cpp: Remove deprecated attribute `Qt::AA_UseHighDpiPixmaps`

`Qt::AA_UseHighDpiPixmaps` is deprecated: High-DPI pixmaps are always enabled. This attribute no longer has any effect.

App/main.cpp

@@ -36,9 +36,6 @@ int main(int argc, char* argv[])
     QLocale::setDefault(QLocale(QLocale::English, QLocale::UnitedStates));
     qRegisterMetaType<QVector<double>>("QVector<double>");
 
-    if (!options.disableHighDPISupport())
-        QCoreApplication::setAttribute(Qt::AA_UseHighDpiPixmaps);
-
     QApplication app(argc, argv);
     QApplication::setApplicationName("BornAgain");
     QApplication::setApplicationVersion(GUI::Util::Path::getBornAgainVersionString());