submodule--helper: remove unused SUPPORT_SUPER_PREFIX flags

submodule--helper: remove unused SUPPORT_SUPER_PREFIX flags

Remove the SUPPORT_SUPER_PREFIX flag from "add", "init" and
"summary". For the "add" command it hasn't been used since [1],
likewise for "init" and "summary" since [2] and [3], respectively.

As implemented in 74866d75793 (git: make super-prefix option,
2016-10-07) the SUPPORT_SUPER_PREFIX flag in git.c applies for the
entire command, but as implemented in 89c86265576 (submodule helper:
support super prefix, 2016-12-08) we assert here in
cmd_submodule__helper() that we're not getting the flag unexpectedly.

1. 8c8195e9c3e (submodule--helper: introduce add-clone subcommand,
   2021-07-10)
2. 6e7c14e65c8 (submodule update --init: display correct path from
   submodule, 2017-01-06)
3. 1cf823d8f00 (submodule: remove unnecessary `prefix` based option
   logic, 2021-06-22)

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