gpiolib: of: remove of_get_gpio[_flags]() and of_get_named_gpio_flags()

There are no more users of these APIs in the mainline kernel, remove
them. This leaves of_get_named_gpio() as the only legacy OF-specific
API.

Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>