gpio: it87: remove unused code

Fix the following clang warning:

drivers/gpio/gpio-it87.c:128:20: warning: unused function 'superio_outw'
[-Wunused-function].

Reported-by: Abaci Robot <abaci@linux.alibaba.com>
Signed-off-by: Jiapeng Chong <jiapeng.chong@linux.alibaba.com>
Acked-by: Simon Guinot <simon.guinot@sequanux.org>
Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>