sandbox_noblk_defconfig: disable CONFIG_GENERIC_MMC

As commit 54925327fa11 ("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>

configs/sandbox_noblk_defconfig

@@ -105,7 +105,6 @@ CONFIG_CROS_EC_SPI=y
 CONFIG_PWRSEQ=y
 CONFIG_SPL_PWRSEQ=y
 # CONFIG_MMC is not set
-CONFIG_GENERIC_MMC=y
 CONFIG_SPI_FLASH_SANDBOX=y
 CONFIG_SPI_FLASH=y
 CONFIG_SPI_FLASH_ATMEL=y