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>
1 file changed