irqchip/irq-omap-intc: Remove omap3_init_irq()

All mach-omap2 variants are device tree only now, so this function is dead
code. Remove it.

Signed-off-by: Ladislav Michl <ladis@linux-mips.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Tony Lindgren <tony@atomide.com>
Cc: Marc Zyngier <marc.zyngier@arm.com>
Cc: linux-omap@vger.kernel.org
Cc: Jason Cooper <jason@lakedaemon.net>
Link: https://lkml.kernel.org/r/20171016160422.uu2i7vvrgy7cc4aw@lenoch