gpio: remove broken irq_to_gpio() interface

gpiolib has removed the irq_to_gpio() API several years ago,
but the global header still provided a non-working stub.

To prevent new users of this broken function from showing
up, let's remove the stubs as well.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>