sandbox_noblk_defconfig: disable CONFIG_GENERIC_MMC

As commit 54925327 ("mmc: move CONFIG_GENERIC_MMC to Kconfig")
addressed, this is one of the last weird defconfigs that define
CONFIG_GENERIC_MMC without CONFIG_MMC.

Now I took a closer look at this.  Given that neither CONFIG_CMD_MMC
nor CONFIG_MMC is set for this defconfig, CONFIG_GENERIC_MMC
should be disabled.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>