board: st: stm32mp1: Clean env_get_location()

ENV_IS_IN_EXT4 flag is no more used in any STM32 defconfig,
remove the related code.

Signed-off-by: Patrice Chotard <[e-mail]>
Reviewed-by: Patrick Delaunay <[e-mail]>

board/st/stm32mp1/stm32mp1.c

···
 	case BOOT_FLASH_EMMC:
 		if (CONFIG_IS_ENABLED(ENV_IS_IN_MMC))
 			return ENVL_MMC;
-		else if (CONFIG_IS_ENABLED(ENV_IS_IN_EXT4))
-			return ENVL_EXT4;
 		else
 			return ENVL_NOWHERE;