gpiolib: Do not unexport GPIO on freeing

Since the legacy exporting is gone with 2f804aca ("gpiolib:
Kill unused GPIOF_EXPORT and Co") there is no need to unexport
GPIO on freeing. Remove that call.

Note, the other users of this functionality do that explicitly,
except one SH and one OMAP boardfile which don't free GPIO anyways,
so it is safe to drop the call.

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