packfile: inline custom read_object()

packfile: inline custom read_object()

When the pack code was split into its own file[1], it got a copy of the
static read_object() function. But there's only one caller here, so we
could just inline it. And it's worth doing so, as the name read_object()
invites comparisons to the public read_object_file(), but the two don't
behave quite the same.

[1] The move happened over several commits, but the relevant one here is
    f1d8130be0 (pack: move clear_delta_base_cache(), packed_object_info(),
    unpack_entry(), 2017-08-18).

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>