Revert "Offscreen: Implement QPlatformBackingStore::toImage"

This reverts commit 77895514d5419b77535de093b544aee30686cd22, which
is in principle correct, but results in XPASS'ing tests in qtdeclarative
that we need to adapt first.

Originally reverted only for 6.3, but now also in dev given the amount of
tests that have a QEXPECT_FAIL for the offscreen platform in qtdeclarative.

Change-Id: Ic914655c737c3b279c14a66220775f3c7a6cdab8
Reviewed-by: Fabian Kosmale <fabian.kosmale@qt.io>
(cherry picked from commit 8498d1a14b5a8392ee6b50a87b82f9d85d13193e)
Reviewed-by: Volker Hilsheimer <volker.hilsheimer@qt.io>

src/plugins/platforms/offscreen/qoffscreencommon.h

@@ -106,7 +106,6 @@ public:
     bool scroll(const QRegion &area, int dx, int dy) override;
 
     QPixmap grabWindow(WId window, const QRect &rect) const;
-    QImage toImage() const override { return m_image; }
 
     static QOffscreenBackingStore *backingStoreForWinId(WId id);