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 <patrice.chotard@foss.st.com>
Reviewed-by: Patrick Delaunay <patrick.delaunay@foss.st.com>
diff --git a/board/st/stm32mp1/stm32mp1.c b/board/st/stm32mp1/stm32mp1.c
index d5e5e77..15fb9e4 100644
--- a/board/st/stm32mp1/stm32mp1.c
+++ b/board/st/stm32mp1/stm32mp1.c
@@ -751,8 +751,6 @@
 	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;