gpio: Remove unused and obsoleted irq_to_gpio()

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>

Documentation/driver-api/gpio/legacy.rst

@@ -387,9 +387,6 @@ map between them using calls like::
 	/* map GPIO numbers to IRQ numbers */
 	int gpio_to_irq(unsigned gpio);
 
-	/* map IRQ numbers to GPIO numbers (avoid using this) */
-	int irq_to_gpio(unsigned irq);
-
 Those return either the corresponding number in the other namespace, or
 else a negative errno code if the mapping can't be done.  (For example,
 some GPIOs can't be used as IRQs.)  It is an unchecked error to use a GPIO
@@ -405,11 +402,6 @@ devices, by the board-specific initialization code.  Note that IRQ trigger
 options are part of the IRQ interface, e.g. IRQF_TRIGGER_FALLING, as are
 system wakeup capabilities.
 
-Non-error values returned from irq_to_gpio() would most commonly be used
-with gpio_get_value(), for example to initialize or update driver state
-when the IRQ is edge-triggered.  Note that some platforms don't support
-this reverse mapping, so you should avoid using it.
-
 
 Emulating Open Drain Signals
 ----------------------------