pinctrl: remove unused pin_is_valid()

This function was used by pin_request() to pointlessly double-check
the pin validity, and it was the only user ever.

Since commit d2f6a1c6 ("pinctrl: remove double pin validity
check."), no one has ever used it.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>