irqchip: Remove sigma tango driver

The tango platform is getting removed, so the driver is no
longer needed.

Cc: Marc Gonzalez <marc.w.gonzalez@free.fr>
Cc: Mans Rullgard <mans@mansr.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Mans Rullgard <mans@mansr.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20210120133008.2421897-2-arnd@kernel.org