builtin/pack-objects.c: avoid redundant NULL check

Before calling `for_each_object_in_pack()`, the caller `read_packs_list_from_stdin()` loops through each of the `include_packs` and checks that its `->util` pointer (which is used to store the `struct packed_git *` itself) is non-NULL. This check is redundant, because `read_packs_list_from_stdin()` already checks that the included packs are non-NULL earlier on in the same function (and it does not add any new entries in between). Remove this check, since it is not doing anything in the meantime. Co-authored-by: Victoria Dye <vdye@github.com> Signed-off-by: Taylor Blau <me@ttaylorr.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>