QSpan: remove old qspan_p.h forwarding header

There are no in-tree users left, which leaves

- QtWebEngine (which is either compatible with 6.5 (which didn't even
  have qspan_p.h) or 6.8 (which has the public qspan.h)) and

- QtCreator, which doesn't use QSpan atm and recently updated its Qt
  version to 6.7 (which has the public qspan.h).

So it should be safe to remove now.

Change-Id: I50d0eec71d794021598ddc24c980f51a900694e8
Reviewed-by: Eike Ziller <eike.ziller@qt.io>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/corelib/CMakeLists.txt

@@ -311,7 +311,6 @@ qt_internal_add_module(Core
         tools/qsize.cpp tools/qsize.h
         tools/qsmallbytearray_p.h
         tools/qspan.h
-        tools/qspan_p.h
         tools/qstack.h
         tools/qtaggedpointer.h
         tools/qtools_p.h

src/corelib/tools/qspan_p.h

@@ -1,24 +0,0 @@
-// Copyright (C) 2023 The Qt Company Ltd.
-// SPDX-License-Identifier: LicenseRef-Qt-Commercial OR LGPL-3.0-only OR GPL-2.0-only OR GPL-3.0-only
-
-#ifndef QSPAN_P_H
-#define QSPAN_P_H
-
-#include <QtCore/qspan.h>
-
-//
-//  W A R N I N G
-//  -------------
-//
-// This file is not part of the Qt API.  It exists purely as an
-// implementation detail.  This header file may change from version to
-// version without notice, or even be removed.
-//
-// We mean it.
-//
-
-QT_BEGIN_NAMESPACE
-
-QT_END_NAMESPACE
-
-#endif // QSPAN_P_H