QMetaObject: clean up includes

Remove duplicate ones and regroup them.

Add a comment explaining why qthread(_p).h isn't guarded by an
`#if QT_CONFIG(thread)`; thanks to Fabian Kosmale for the explanation.

Pick-to: 6.7
Change-Id: I9b13fd62ab92fa6d2ffc4d004d77e95e95f5fb08
Reviewed-by: Fabian Kosmale <fabian.kosmale@qt.io>

src/corelib/kernel/qmetaobject.cpp

@@ -3,26 +3,23 @@
 // SPDX-License-Identifier: LicenseRef-Qt-Commercial OR LGPL-3.0-only OR GPL-2.0-only OR GPL-3.0-only
 
 #include "qmetaobject.h"
-#include "qmetatype.h"
-#include "qobject.h"
 #include "qmetaobject_p.h"
+#include "qmetatype.h"
 #include "qmetatype_p.h"
+#include "qobject.h"
+#include "qobject_p.h"
 
 #include <qcoreapplication.h>
-#include <qcoreevent.h>
-#include <qdatastream.h>
-#include <qstringlist.h>
-#include <qthread.h>
 #include <qvariant.h>
-#include <qdebug.h>
+
+// qthread(_p).h uses QT_CONFIG(thread) internally and has a dummy
+// interface for the non-thread support case
+#include <qthread.h>
+#include "private/qthread_p.h"
 #if QT_CONFIG(thread)
 #include <qsemaphore.h>
 #endif
 
-#include "private/qobject_p.h"
-#include "private/qmetaobject_p.h"
-#include "private/qthread_p.h"
-
 // for normalizeTypeInternal
 #include "private/qmetaobject_moc_p.h"