power: supply: z2_battery: Drop unused variable

The kernel test robot reports an unused variable in the
Z2 battery code caused by a recent commit.

Fixes: a3b4388e ("power: supply: z2_battery: Convert to GPIO descriptors")
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>