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