git-submodule.sh: remove unused $prefix variable

git-submodule.sh: remove unused $prefix variable

Remove the $prefix variable which isn't used anymore, and hasn't been
since b3c5f5cb048 (submodule: move core cmd_update() logic to C,
2022-03-15).

Before that we'd use it to invoke "git submodule--helper" with the
"--recursive-prefix" option, but since b3c5f5cb048 that "git
submodule--helper" option is only used when it invokes itself.

So the "--recursive-prefix" option is still in use, but at this point
only when the helper invokes itself during submodule recursion. See
the "--recursive-prefix" option added in
c51f8f94e5b (submodule--helper: run update procedures from C,
2021-08-24).

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