doc: Remove obsolete note for QWidget::paintEvent()

qt_x11_set_global_double_buffer() is dead code since Qt 5.

Change-Id: Ie9a33b6f03dc2e39f12bc790292bb0d227f05c44
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@qt.io>

src/widgets/doc/snippets/code/src_gui_kernel_qwidget.cpp

@@ -127,11 +127,3 @@ setUpdatesEnabled(false);
 bigVisualChanges();
 setUpdatesEnabled(true);
 //! [13]
-
-
-//! [14]
-...
-extern void qt_x11_set_global_double_buffer(bool);
-qt_x11_set_global_double_buffer(false);
-...
-//! [14]

src/widgets/kernel/qwidget.cpp

@@ -9597,11 +9597,6 @@ void QWidget::leaveEvent(QEvent *)
     Since Qt 4.0, QWidget automatically double-buffers its painting, so there
     is no need to write double-buffering code in paintEvent() to avoid flicker.
 
-    \b{Note for the X11 platform}: It is possible to toggle global double
-    buffering by calling \c qt_x11_set_global_double_buffer(). For example,
-
-    \snippet code/src_gui_kernel_qwidget.cpp 14
-
     \note Generally, you should refrain from calling update() or repaint()
     \b{inside} a paintEvent(). For example, calling update() or repaint() on
     children inside a paintevent() results in undefined behavior; the child may