gpio: Remove legacy API documentation

In order to discourage people to use old and legacy GPIO APIs
remove the respective documentation completely. It also helps
further cleanups of the legacy GPIO API leftovers, which is
ongoing task.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Hu Haowen <2023002089@link.tyut.edu.cn>
Link: https://lore.kernel.org/r/20240508101703.830066-1-andriy.shevchenko@linux.intel.com


Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>