Remove outdated HACKING file

There is no bluedevil coding style documented
We use SPDX identifiers now

HACKING

-Please follow the bluedevil style before submitting patches.
-
-All copyright headers are unified with the following style:
-
-/*****************************************************************************
- * This file is part of the KDE project                                      *
- *                                                                           *
- * Copyright (C) 2010 AuthorName Surname Surname <author@server.org>         *
- *                                                                           *
- * This library is free software; you can redistribute it and/or             *
- * modify it under the terms of the GNU Library General Public               *
- * License as published by the Free Software Foundation; either              *
- * version 2 of the License, or (at your option) any later version.          *
- *                                                                           *
- * This library is distributed in the hope that it will be useful,           *
- * but WITHOUT ANY WARRANTY; without even the implied warranty of            *
- * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU         *
- * Library General Public License for more details.                          *
- *                                                                           *
- * You should have received a copy of the GNU Library General Public License *
- * along with this library; see the file COPYING.LIB.  If not, write to      *
- * the Free Software Foundation, Inc., 51 Franklin Street, Fifth Floor,      *
- * Boston, MA 02110-1301, USA.                                               *
- *****************************************************************************/