pinctrl: sh-pfc: Remove empty gpio_function_free()

gpio_chip.free() is optional, and can just be left unimplemented.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Acked-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Acked-by: Linus Walleij <linus.walleij@linaro.org>