gpio: drop broken to_gpio_irq_chip() helper

Drop the broken to_gpio_irq_chip() container_of() helper, which would
break the build for anyone who tries to use it.

Specifically, struct gpio_irq_chip only holds a pointer to a struct
irq_chip so using container_of() on an irq-chip pointer makes no sense.

Fixes: da80ff81

 ("gpio: Move irqchip into struct gpio_irq_chip")
Cc: Thierry Reding <treding@nvidia.com>
Cc: Grygorii Strashko <grygorii.strashko@ti.com>
Signed-off-by: Johan Hovold <johan@kernel.org>
Reviewed-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>