nios2: remove ARCH_WANT_OPTIONAL_GPIOLIB

This symbols is not needed to get access to selecting the
GPIOLIB anymore: any arch can select GPIOLIB.

Cc: Michael Büsch <m@bues.ch>
Cc: nios2-dev@lists.rocketboards.org
Acked-by: Ley Foon Tan <lftan@altera.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>