submodule-config.h: remove unused SUBMODULE_INIT macro

This macro was added and used in c68f837576 (implement fetching of
moved submodules, 2017-10-16) but its last user went away in
be76c21282 (fetch: ensure submodule objects fetched, 2018-12-06).

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

submodule-config.h

@@ -45,10 +45,6 @@ struct submodule {
 	struct object_id gitmodules_oid;
 	int recommend_shallow;
 };
-
-#define SUBMODULE_INIT { NULL, NULL, NULL, RECURSE_SUBMODULES_NONE, \
-	NULL, NULL, SUBMODULE_UPDATE_STRATEGY_INIT, { { 0 } }, -1 };
-
 struct submodule_cache;
 struct repository;