MixxxApplication: do not build hacks for touch input with Qt6

These hacks were to simulate mouse input with touch input
with the legacy QWidgets GUI. They do not build with Qt6 and
are not needed with QML.

src/mixxxapplication.cpp

@@ -40,6 +40,7 @@ Q_IMPORT_PLUGIN(QJpegPlugin)
 Q_IMPORT_PLUGIN(QGifPlugin)
 #endif // QT_STATIC
 
+#if QT_VERSION < QT_VERSION_CHECK(6, 0, 0)
 namespace {
 
 /// This class allows to change the button of a mouse event on the fly.
@@ -53,6 +54,7 @@ class QMouseEventEditable : public QMouseEvent {
 };
 
 } // anonymous namespace
+#endif
 
 MixxxApplication::MixxxApplication(int& argc, char** argv)
         : QApplication(argc, argv),
@@ -103,6 +105,7 @@ void MixxxApplication::registerMetaTypes() {
     qRegisterMetaType<mixxx::FileInfo>("mixxx::FileInfo");
 }
 
+#if QT_VERSION < QT_VERSION_CHECK(6, 0, 0)
 bool MixxxApplication::notify(QObject* target, QEvent* event) {
     // All touch events are translated into two simultaneous events: one for
     // the target QWidgetWindow and one for the target QWidget.
@@ -147,3 +150,4 @@ bool MixxxApplication::touchIsRightButton() {
     }
     return m_pTouchShift->toBool();
 }
+#endif