selftests: harness: remove unneeded __constructor_order_last()

__constructor_order_last() is unneeded.

If __constructor_order_last() is not called on backward-order systems,
__constructor_order will remain 0 instead of being set to
_CONSTRUCTOR_ORDER_BACKWARD (= -1).

__LIST_APPEND() will still take the 'else' branch, so there is no
difference in the behavior.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>