selftests/kexec: remove broken EFI_VARS secure boot fallback check

Commit b433a52a ("selftests/kexec: update get_secureboot_mode")
refactored the code that discovers the EFI secure boot mode so it only
depends on either the efivars pseudo filesystem or the efivars sysfs
interface, but never both.

However, the latter version was not implemented correctly, given the
fact that the local 'efi_vars' variable never assumes a value. This
means the fallback has been dead code ever since it was introduced.

So let's drop the fallback altogether. The sysfs interface has been
deprecated for ~10 years now, and is only enabled on x86 to begin with,
so it is time to get rid of it entirely.

Reviewed-by: Mimi Zohar <zohar@linux.ibm.com>
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>