gpio: Remove unused and obsoleted gpio_export_link()

gpio_export_link() is legacy and unused API, remove it for good.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: Yanteng Si <siyanteng@loongson.cn>
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>