files-backend.c: delete dead code in files_ref_iterator_begin()

It's not in the diff context, but files_downcast() is called before this
check. If "refs" is NULL, we would have segfaulted before reaching the
check here. And we should never see NULL refs in backend code (frontend
should have caught it).
Signed-off-by: NNguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

refs/files-backend.c

@@ -1897,9 +1897,6 @@ static struct ref_iterator *files_ref_iterator_begin(
 	struct files_ref_iterator *iter;
 	struct ref_iterator *ref_iterator;
 
-	if (!refs)
-		return empty_ref_iterator_begin();
-
 	if (ref_paranoia < 0)
 		ref_paranoia = git_env_bool("GIT_REF_PARANOIA", 0);
 	if (ref_paranoia)