gpiolib: remove unused gpio_cansleep()

There is not a single user in the entire kernel of this deprecated API,
kill it for good.

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