sgi/secure_boot: remove unused definition MM_SECURE_STORAGE_ENABLE

The definition MM_SECURE_STORAGE_ENABLE isn't used for the secure
storage support in edk2-platforms. So remove this definition.

Signed-off-by: Thomas Abraham <thomas.abraham@arm.com>
Change-Id: I01fb01cbb34b99750ba5cdd331474dc3418da8c9