sh-pfc: sh73a0: Remove USB function GPIOS

All sh73a0 platforms now use the pinctrl API to control the USB pins,
the corresponding function GPIOS are unused. Remove them.

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