serial: sifive: Remove useless license text when SPDX-License-Identifier is already used

An SPDX-License-Identifier is already in place. There is no need to
duplicate part of the corresponding license.
Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Link: https://lore.kernel.org/r/dbc54eff6f4e077d9126054f395d5bc5b3405917.1654936915.git.christophe.jaillet@wanadoo.frSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/sifive.c

@@ -4,16 +4,6 @@
  * Copyright (C) 2018 Paul Walmsley <paul@pwsan.com>
  * Copyright (C) 2018-2019 SiFive
  *
- * This program is free software; you can redistribute it and/or modify
- * it under the terms of the GNU General Public License as published by
- * the Free Software Foundation; either version 2 of the License, or
- * (at your option) any later version.
- *
- * This program 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 General Public License for more details.
- *
  * Based partially on:
  * - drivers/tty/serial/pxa.c
  * - drivers/tty/serial/amba-pl011.c