submodule: remove sh function in favor of helper

submodule: remove sh function in favor of helper

Remove the now-redundant "get_default_remote" function by converting
its last user to the "print-default-remote" helper.

As can be seen in 13424764db ("submodule: port submodule subcommand
'sync' from shell to C", 2018-01-15) this helper is already used
internally by the C code for submodule remote name discovery.

The "get_default_remote" function in "git-parse-remote.sh" will be
removed in a follow-up change.

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