git-submodule.sh: remove unused sanitize_submodule_env()

git-submodule.sh: remove unused sanitize_submodule_env()

The sanitize_submodule_env() function was last used before
b3c5f5cb048 (submodule: move core cmd_update() logic to C,
2022-03-15), let's remove it.

This also allows us to remove clear_local_git_env() from
git-sh-setup.sh. That function hasn't been documented in
Documentation/git-sh-setup.sh, and since 14111fc4927 (git: submodule
honor -c credential.* from command line, 2016-02-29) it had only been
used in the sanitize_submodule_env() function being removed here.

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