irqchip: mips-gic: Remove gic_read_local_vp_id()

Nothing needs gic_read_local_vp_id() any longer, so remove the dead
code.

Signed-off-by: Paul Burton <paul.burton@imgtec.com>
Acked-by: Marc Zyngier <marc.zyngier@arm.com>
Cc: Jason Cooper <jason@lakedaemon.net>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/17024/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>