genirq: Remove preflow handler support

That was put in place for sparc64, and blackfin also used it for some time;
sparc64 no longer uses those, and blackfin is dead.

As there are no more users, remove preflow handlers.

Signed-off-by: Valentin Schneider <valentin.schneider@arm.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lkml.kernel.org/r/20200703155645.29703-3-valentin.schneider@arm.com