cl-som-imx7: Remove CONFIG_SYS_MMCSD_RAW_MODE_U_BOOT_USE_SECTOR=y

Since commit 1da1938d ("spl: Add default values for ARCH_MX7")
CONFIG_SYS_MMCSD_RAW_MODE_U_BOOT_USE_SECTOR is selected by default on
i.MX7 platforms, so remove it from the board defconfig.

Signed-off-by: Fabio Estevam <fabio.estevam@nxp.com>