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

This leaves gpio_to_irq() as it's used yet in many places.
Nevertheless, removal of its counterpart is a good signal
to whoever even trying to consider using them that do not.
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Acked-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>