odb: drop unused `for_each_{loose,packed}_object()` functions

odb: drop unused `for_each_{loose,packed}_object()` functions

We have converted all callers of `for_each_loose_object()` and
`for_each_packed_object()` to use their new replacement functions
instead. We can thus remove them now.

Do so and inline `packfile_store_for_each_object_internal()` now that it
only has a single callsite again. This makes it a bit easier to follow
the callback indirection that is happening there.

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