builtin/gc: convert to use `packfile_store_get_all_packs()`

builtin/gc: convert to use `packfile_store_get_all_packs()`

When running maintenance tasks via git-maintenance(1) we have a couple
of auto-conditions that check whether or not a specific task should be
running. One such check is for incremental repacks, which essentially
use `git multi-pack-index repack` to repack a set of smaller packfiles
into one larger packfile.

The auto-condition for this task checks how many packfiles there are
that aren't indexed by any multi-pack index. If there is a sufficient
number then we execute the above command to combine those into a single
pack and add that pack to the MIDX.

As we don't care about MIDX'd packs we use `packfile_store_get_packs()`,
which knows to not load any packs that are indexed by a MIDX. But as
explained in the preceding commit, we want to get rid of that function.

We already handle packfiles that have a MIDX by the very nature of this
function, as we explicitly count non-MIDX'd packs. As such, we can
trivially switch over to use `packfile_store_get_all_packs()` instead.

Do so.

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