Revert "Ensure Qt::WA_Mapped is set in case of obscured native windows."

Qt::WA_Mapped maps (sic) to windowHandle()->isExposed(), and we set/update
it in QWidgetWindow::handleExposeEvent(). Setting it directly in show_sys
shortcuts QPA and assumes showing a window is synchronous on all platforms,
resulting in trying to flush the widget backingstore when the window was
not exposed yet (due to discardSyncRequest starting to return false).

This reverts commit 829b1d13b225e87b8a385397e2b53c7a9f8cda9e.

Change-Id: I0bd700d4939bc69ba184d8586435b68ec3dd72fb
Reviewed-by: Gunnar Sletta <gunnar.sletta@digia.com>