irqchip: Remove metag irqchip drivers

Now that arch/metag/ has been removed, remove the two metag irqchip
drivers. They are of no value without the architecture code.
 - irq-metag: Meta internal (HWSTATMETA) interrupt code.
 - irq-metag-ext: Meta External interrupt code.

Signed-off-by: James Hogan <jhogan@kernel.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Jason Cooper <jason@lakedaemon.net>
Cc: Marc Zyngier <marc.zyngier@arm.com>
Cc: linux-metag@vger.kernel.org