arm64: Remove ununsed set_irq_flags

Now that all users of set_irq_flags and custom flags are converted to
genirq functions, the ARM specific set_irq_flags can be removed.

Signed-off-by: Rob Herring <robh@kernel.org>
Acked-by: Catalin Marinas <catalin.marinas@arm.com>
Cc: linux-arm-kernel@lists.infradead.org
Cc: Russell King <linux@arm.linux.org.uk>
Cc: Will Deacon <will.deacon@arm.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>