Revert "Fix export of QDeferredDeleteEvent, should be Q_CORE_EXPORT"

This reverts commit bc3b18764a.

Reason for revert: QDeferredDeleteEvent changed the size in 6.7. It is
more important to protect the casual user that relied on the mention of
the otherwise undocumented class in the QEvent::Type::DeferredDelete
documentation than to be convenient for one tool, developed by experts
who know how to replace their dynamic_cast with a type() check and a
static_cast and/or extract information from an unexported class.

In fact, not even the autotest-export is needed, as there are no
mentions of the class outside of comments outside QtCore. 

Found in API-review.

[ChangeLog][QtCore][QDeferredDeleteEvent] Made this undocumented class
private and unexported. You will still be able to see the definition in
qcoreevent_p.h, but you won't be able to create objects of the class
anymore.

Change-Id: I8a47c69d356a0bef260e7987bc4eab96430e8072
Pick-to: 6.7
Reviewed-by: David Faure <david.faure@kdab.com>
Reviewed-by: Volker Hilsheimer <volker.hilsheimer@qt.io>

src/corelib/kernel/qcoreevent_p.h

@@ -21,8 +21,7 @@ QT_BEGIN_NAMESPACE
 
 class QCoreApplication;
 
-// GammaRay needs this to be exported with Q_CORE_EXPORT
-class Q_CORE_EXPORT QDeferredDeleteEvent : public QEvent
+class QDeferredDeleteEvent : public QEvent
 {
     Q_DECL_EVENT_COMMON(QDeferredDeleteEvent)
 public: