Remove assertion in QAndroidPlatformScreen::addWindow()

QAndroidPlatformScreen::addWindow() asserted the existence of a backing
store, when a window is added. But QQuickWindow neither needs nor has a
backing store.

This patch removes the assertion, which was introduced in
dbb072eb28

Task-number: QTBUG-97482
Pick-to: 6.6 6.5 6.2 5.15
Change-Id: Ifb43f83d39e16b972754e86d0ff9633f58dac3f8
Reviewed-by: Shawn Rutledge <shawn.rutledge@qt.io>

src/plugins/platforms/android/qandroidplatformscreen.cpp

@@ -172,7 +172,6 @@ bool QAndroidPlatformScreen::event(QEvent *event)
 
 void QAndroidPlatformScreen::addWindow(QAndroidPlatformWindow *window)
 {
-    Q_ASSERT(window->backingStore());
     if (window->parent() && window->isRaster())
         return;