submodule--helper: don't redundantly check "else if (res)"

submodule--helper: don't redundantly check "else if (res)"

The "res" variable must be true at this point in update_submodule(),
as just a few lines above this we've unconditionally:

	if (!res)
		return 0;

So we don't need to guard the "return 1" with an "else if (res)", we
can return unconditionally at this point. See b3c5f5cb048 (submodule:
move core cmd_update() logic to C, 2022-03-15) for the initial
introduction of this code, this check of "res" has always been
redundant.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Reviewed-by: Glen Choo <chooglen@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>