block: Cleanup license notice

Remove the imprecise and sloppy:

  "This files is licensed under the GPL."

license notice in the top level comment.

1) The file already contains a SPDX license identifier which clearly
   states that the license of the file is GPL V2 only

2) The notice resolves to GPL v1 or later for scanners which is just
   contrary to the intent of SPDX identifiers to provide clear and non
   ambiguous license information. Aside of that the value add of this
   notice is below zero,

Cc: Damien Le Moal <damien.lemoal@wdc.com>
Cc: Matias Bjorling <mb@lightnvm.io>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: linux-block@vger.kernel.org
Fixes: 6a5ac984 ("block: Make struct request_queue smaller for CONFIG_BLK_DEV_ZONED=n")
Reviewed-by: Bart Van Assche <bvanassche@acm.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

block/blk-mq-debugfs-zoned.c

 // SPDX-License-Identifier: GPL-2.0
 /*
  * Copyright (C) 2017 Western Digital Corporation or its affiliates.
- *
- * This file is released under the GPL.
  */
 
 #include <linux/blkdev.h>