CMD: Drop CONFIG_CMD_EMMC, it's not used

$ git grep CMD_EMMC
include/configs/u8500_href.h:#define CONFIG_CMD_EMMC

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: dirk.behme@googlemail.com
Cc: helmut.raiger@hale.at
Cc: john.rigby@linaro.org
Cc: vapier@gentoo.org