git-submodule.sh: use "$quiet", not "$GIT_QUIET"

git-submodule.sh: use "$quiet", not "$GIT_QUIET"

Remove the use of the "$GIT_QUIET" variable in favor of our own
"$quiet", ever since b3c5f5cb048 (submodule: move core cmd_update()
logic to C, 2022-03-15) we have not used the "say" function in
git-sh-setup.sh, which is the only thing that's affected by using
"GIT_QUIET".

We still want to support --quiet for our own use though, but let's use
our own variable for that. Now it's obvious that we only care about
passing "--quiet" to "git submodule--helper", and not to change the
output of any "say" invocation.

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