gpio: idio-16: Remove unused legacy interface

All idio-16 library consumers have migrated to the new interface
leveraging the gpio-regmap API. Legacy interface functions and code are
removed as no longer needed.

Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Link: https://lore.kernel.org/r/651cff1cc3eb57b455a8048121cf6a4d4367f018.1680618405.git.william.gray@linaro.org/


Signed-off-by: William Breathitt Gray <william.gray@linaro.org>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>