Windows QPA: remove long-time dead code

It has been commented out since at least 5.3 (the earliest version
hosted on the GitHub mirror), so it seems this code is not used
by anyone for more than 10 years. We can safely remove them.

Change-Id: Ia475ce5a5fdd6db48f415b3647677a35c79bfc5d
Reviewed-by: Oliver Wolff <oliver.wolff@qt.io>

src/plugins/platforms/windows/qwindowswindow.cpp

@@ -794,15 +794,8 @@ void WindowCreationData::fromWindow(const QWindow *w, const Qt::WindowFlags flag
         style = WS_CHILD;
     }
 
-    // if (!testAttribute(Qt::WA_PaintUnclipped))
-    // ### Commented out for now as it causes some problems, but
-    // this should be correct anyway, so dig some more into this
-#ifdef Q_FLATTEN_EXPOSE
-    if (windowIsOpenGL(w)) // a bit incorrect since the is-opengl status may change from false to true at any time later on
-        style |= WS_CLIPSIBLINGS | WS_CLIPCHILDREN; // see SetPixelFormat
-#else
     style |= WS_CLIPSIBLINGS | WS_CLIPCHILDREN ;
-#endif
+
     if (topLevel) {
         if ((type == Qt::Window || dialog || tool)) {
             if (!(flags & Qt::FramelessWindowHint)) {
@@ -2473,11 +2466,7 @@ void QWindowsWindow::setWindowState_sys(Qt::WindowStates newState)
 
     if (stateChange & Qt::WindowFullScreen) {
         if (newState & Qt::WindowFullScreen) {
-#ifndef Q_FLATTEN_EXPOSE
             UINT newStyle = WS_CLIPCHILDREN | WS_CLIPSIBLINGS | WS_POPUP;
-#else
-            UINT newStyle = WS_POPUP;
-#endif
             // Save geometry and style to be restored when fullscreen
             // is turned off again, since on Windows, it is not a real
             // Window state but emulated by changing geometry and style.