Documentation: gpio: Remove text about ARCH_NR_GPIOS

ARCH_NR_GPIOS have been removed, clean up the documentation.

After this patch, the only place when ARCH_NR_GPIOS remains is in
translations/zh_CN/gpio.txt and translations/zh_TW/gpio.txt.
I don't have the skills to update that, anyway those two files are
already out of sync as they are still mentionning ARCH_REQUIRE_GPIOLIB
which was removed by commit 65053e1a ("gpio: delete
ARCH_[WANTS_OPTIONAL|REQUIRE]_GPIOLIB")
Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>

Documentation/driver-api/gpio/legacy.rst

@@ -558,11 +558,6 @@ Platform Support
 To force-enable this framework, a platform's Kconfig will "select" GPIOLIB,
 else it is up to the user to configure support for GPIO.
 
-It may also provide a custom value for ARCH_NR_GPIOS, so that it better
-reflects the number of GPIOs in actual use on that platform, without
-wasting static table space.  (It should count both built-in/SoC GPIOs and
-also ones on GPIO expanders.
-
 If neither of these options are selected, the platform does not support
 GPIOs through GPIO-lib and the code cannot be enabled by the user.