genirq/irq_sim: Remove the license boilerplate

There is the SPDX license identifier now in the irq simulator. Remove the
license boilerplate.

While at it: update the copyright notice, since I did some changes in 2018.
Signed-off-by: Bartosz Golaszewski <brgl@bgdev.pl>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lkml.kernel.org/r/20180426200747.8344-1-brgl@bgdev.pl

kernel/irq/irq_sim.c

 // SPDX-License-Identifier: GPL-2.0+
 /*
- * Copyright (C) 2017 Bartosz Golaszewski <brgl@bgdev.pl>
- *
- * 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.
+ * Copyright (C) 2017-2018 Bartosz Golaszewski <brgl@bgdev.pl>
  */
 
 #include <linux/slab.h>