Remove warning about being unable to instantiate accessibility interface

This warning was a good idea when the plugins were needed to give a hint
where to start debugging. Now the a11y plugins were merged into
qtwidgets/qtquick, so there is no more need for this warning which may
pop up on custom widgets e.g. inheriting QWindow.

Task-number: QTBUG-50215
Change-Id: Ic5efbb3dfbd07a7139884126a2604b54c99005e4
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@theqtcompany.com>
Reviewed-by: Morten Johan Sørvig <morten.sorvig@theqtcompany.com>

src/gui/accessible/qaccessible.cpp

@@ -1758,15 +1758,8 @@ QAccessibleInterface *QAccessibleEvent::accessibleInterface() const
         return QAccessible::accessibleInterface(m_uniqueId);
 
     QAccessibleInterface *iface = QAccessible::queryAccessibleInterface(m_object);
-    if (!iface || !iface->isValid()) {
-        static bool hasWarned = false;
-        if (!iface && !hasWarned) {
-            qWarning() << "Problem creating accessible interface for: " << m_object << endl
-                       << "Make sure to deploy Qt with accessibility plugins.";
-            hasWarned = true;
-        }
+    if (!iface || !iface->isValid())
         return 0;
-    }
 
     if (m_child >= 0) {
         QAccessibleInterface *child = iface->child(m_child);