power: remove z2_battery driver

The PXA z2 platform is gone, and this driver is now orphaned.

Cc: linux-pm@vger.kernel.org
Cc: Peter Edwards <sweetlilmre@gmail.com>
Acked-by: Sebastian Reichel <sebastian.reichel@collabora.com>
Acked-by: Robert Jarzmik <robert.jarzmik@free.fr>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>