configs: dra7xx_evm: Remove ENV_IS_IN_FAT

With U-boot supporting environment in multiple places, enable only
ENV_IS_IN_EMMC

Signed-off-by: Faiz Abbas <faiz_abbas@ti.com>
Reviewed-by: Lokesh Vutla <lokeshvutla@ti.com>
Reviewed-by: Tom Rini <trini@konsulko.com>