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>

git-submodule.sh

@@ -578,7 +578,7 @@ cmd_update()
 				fetch_in_submodule "$sm_path" $depth ||
 				die "$(eval_gettext "Unable to fetch in submodule path '\$sm_path'")"
 			fi
-			remote_name=$(sanitize_submodule_env; cd "$sm_path" && get_default_remote)
+			remote_name=$(sanitize_submodule_env; cd "$sm_path" && git submodule--helper print-default-remote)
 			sha1=$(sanitize_submodule_env; cd "$sm_path" &&
 				git rev-parse --verify "${remote_name}/${branch}") ||
 			die "$(eval_gettext "Unable to find current \${remote_name}/\${branch} revision in submodule path '\$sm_path'")"