gpio: remove of_get_named_gpio() and <linux/of_gpio.h>

gpio: remove of_get_named_gpio() and <linux/of_gpio.h>

All in-tree consumers have been converted to the descriptor-based API.
Remove the deprecated of_get_named_gpio() helper, delete the
<linux/of_gpio.h> header, and drop the corresponding entry from
MAINTAINERS.

Also remove the completed TODO item for this cleanup.

Signed-off-by: Jialu Xu <xujialu@vimux.org>
Reviewed-by: Linus Walleij <linusw@kernel.org>
Link: https://patch.msgid.link/02ABDA1F9E3FAF1F+20260307030623.3495092-6-xujialu@vimux.org
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@oss.qualcomm.com>