rockchip: rk3188: drop CONFIG_SYS_NO_FLASH

Commit e856bdcf ("flash: complete CONFIG_SYS_NO_FLASH move with renaming")
obsoleted the CONFIG_SYS_NO_FLASH option, which still is in our
rk3188_common.h header, resulting in warnings like
    The following new ad-hoc CONFIG options were detected:
    CONFIG_SYS_NO_FLASH

So also drop it from the rk3188 header.

Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Acked-by: Simon Glass <sjg@chromium.org>