submodule: remove the per-repo cache

Having this cache and giving them out goes against our multithreading
guarantees and it makes it impossible to use submodules in a
multi-threaded environment, as any thread can ask for a refresh which
may reallocate some string in the submodule struct which we've accessed
in a different one via a getter.

This makes the submodules behave more like remotes, where each object is
created upon request and not shared except explicitly by the user. This
means that some tests won't pass yet, as they assume they can affect the
submodule objects in the cache and that will affect later operations.

src/checkout.c

@@ -1860,11 +1860,6 @@ static int checkout_create_submodules(
 	git_diff_delta *delta;
 	size_t i;
 
-	/* initial reload of submodules if .gitmodules was changed */
-	if (data->reload_submodules &&
-		(error = git_submodule_reload_all(data->repo, 1)) < 0)
-		return error;
-
 	git_vector_foreach(&data->diff->deltas, i, delta) {
 		if (actions[i] & CHECKOUT_ACTION__DEFER_REMOVE) {
 			/* this has a blocker directory that should only be removed iff
@@ -1885,8 +1880,7 @@ static int checkout_create_submodules(
 		}
 	}
 
-	/* final reload once submodules have been updated */
-	return git_submodule_reload_all(data->repo, 1);
+	return 0;
 }
 
 static int checkout_lookup_head_tree(git_tree **out, git_repository *repo)

src/repository.c

@@ -109,7 +109,6 @@ void git_repository__cleanup(git_repository *repo)
 
 	git_cache_clear(&repo->objects);
 	git_attr_cache_flush(repo);
-	git_submodule_cache_free(repo);
 
 	set_config(repo, NULL);
 	set_index(repo, NULL);

src/repository.h

@@ -121,7 +121,6 @@ struct git_repository {
 	git_refdb *_refdb;
 	git_config *_config;
 	git_index *_index;
-	git_submodule_cache *_submodules;
 
 	git_cache objects;
 	git_attr_cache *attrcache;