midx: remove now-unused linked list of multi-pack indices

In the preceding commits we have migrated all users of the linked list
of multi-pack indices to instead use those stored in the object database
sources. Remove those now-unused pointers.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

midx.h

@@ -35,8 +35,6 @@ struct odb_source;
 	"GIT_TEST_MULTI_PACK_INDEX_WRITE_INCREMENTAL"
 
 struct multi_pack_index {
-	struct multi_pack_index *next;
-
 	const unsigned char *data;
 	size_t data_len;