gpio: remove unused logging helpers

The general rule of the kernel is to not provide symbols that have no
users upstream. Let's remove logging helpers that are not used anywhere.

This will save us work later when we'll be modifying them to use the
upcoming SRCU infrastructure.

Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Acked-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>