genirq: Remove license boilerplate/references

Now that SPDX identifiers are in place, remove the boilerplate or
references.

The change in timings.c has been acked by the author.
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Cc: Kate Stewart <kstewart@linuxfoundation.org>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Philippe Ombredanne <pombredanne@nexb.com>
Link: https://lkml.kernel.org/r/20180314212030.668321222@linutronix.de

kernel/irq/debugfs.c

 // SPDX-License-Identifier: GPL-2.0
-/*
- * Copyright 2017 Thomas Gleixner <tglx@linutronix.de>
- *
- * This file is licensed under the GPL V2.
- */
+// Copyright 2017 Thomas Gleixner <tglx@linutronix.de>
+
 #include <linux/irqdomain.h>
 #include <linux/irq.h>
 #include <linux/uaccess.h>

kernel/irq/timings.c

 // SPDX-License-Identifier: GPL-2.0
-/*
- * Copyright (C) 2016, Linaro Ltd - Daniel Lezcano <daniel.lezcano@linaro.org>
- *
- * This program is free software; you can redistribute it and/or modify
- * it under the terms of the GNU General Public License version 2 as
- * published by the Free Software Foundation.
- *
- */
+// Copyright (C) 2016, Linaro Ltd - Daniel Lezcano <daniel.lezcano@linaro.org>
+
 #include <linux/kernel.h>
 #include <linux/percpu.h>
 #include <linux/slab.h>