git-submodule.sh: remove unused top-level "--branch" argument

git-submodule.sh: remove unused top-level "--branch" argument

In 5c08dbbdf1a (git-submodule: fix subcommand parser, 2008-01-15) the
"--branch" option was supported as an option to "git submodule"
itself, i.e. "git submodule --branch" as a side-effect of its
implementation.

Then in b57e8119e6e (submodule: teach set-branch subcommand,
2019-02-08) when the "set-branch" subcommand was added the assertion
that we shouldn't have "--branch" anywhere except as an argument to
"add" and "set-branch" was copy/pasted from the adjacent check for
"--cache" added (or rather modified) in 496eeeb19b9 (git-submodule.sh:
avoid "test <cond> -a/-o <cond>", 2014-06-10).

But there's been a logic error in that check, which at a glance looked
like it should be supporting:

    git submodule --branch <branch> (add | set-branch) [<options>]

But due to "||" in the condition (as opposed to "&&" for "--cache") if
we have "--branch" here already we'll emit usage, even for "add" and
"set-branch".

So in addition to never having documented this form, it hasn't worked
since b57e8119e6e was released with v2.22.0.

So it's safe to remove this code. I.e. we don't want to support the
form noted above, but only:

    git submodule (add | set-branch) --branch <branch> [<options>]

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