midx: stop using linked list when closing MIDX

midx: stop using linked list when closing MIDX

When calling `close_midx()` we not only close the multi-pack index for
one object source, but instead we iterate through the whole linked list
of MIDXs to close all of them. This linked list is about to go away in
favor of using the new per-source pointer to its respective MIDX.

Refactor the function to iterate through sources instead.

Note that after this patch, there's a couple of callsites left that
continue to use `close_midx()` without iterating through all sources.
These are all cases where we don't care about the MIDX from other
sources though, so it's fine to keep them as-is.

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