gpio: Drop the chained IRQ handler assign function

gpiochip_set_chained_irqchip() would assign a chained handler
to a GPIO chip. We now populate struct gpio_irq_chip for all
chained GPIO irqchips so drop this function.

Cc: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Link: https://lore.kernel.org/r/20200113220800.77817-1-linus.walleij@linaro.org
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>

Documentation/driver-api/gpio/driver.rst

@@ -507,11 +507,6 @@ available but we try to move away from this:
   cascaded irq has to be handled by a threaded interrupt handler.
   Apart from that it works exactly like the chained irqchip.
 
-- DEPRECATED: gpiochip_set_chained_irqchip(): sets up a chained cascaded irq
-  handler for a gpio_chip from a parent IRQ and passes the struct gpio_chip*
-  as handler data. Notice that we pass is as the handler data, since the
-  irqchip data is likely used by the parent irqchip.
-
 - gpiochip_set_nested_irqchip(): sets up a nested cascaded irq handler for a
   gpio_chip from a parent IRQ. As the parent IRQ has usually been
   explicitly requested by the driver, this does very little more than