gpio: mockup: remove unused variable gc

The variable gc is assigned but never read and is redundant. Remove it.
Cleans up clang warning:

drivers/gpio/gpio-mockup.c:169:2: warning: Value stored to 'gc' is never read

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>