genirq/irqdomain: Remove WARN_ON() on out-of-memory condition

There is no need to print a backtrace when memory allocation fails, as
the memory allocation core already takes care of that.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Marc Zyngier <marc.zyngier@arm.com>
Link: https://lkml.kernel.org/r/20190527115742.2693-1-geert+renesas@glider.be