packfile: drop nth_packed_object_sha1()

packfile: drop nth_packed_object_sha1()

Once upon a time, nth_packed_object_sha1() was the primary way to get
the oid of a packfile's index position. But these days we have the more
type-safe nth_packed_object_id() wrapper, and all callers have been
converted.

Let's drop the "sha1" version (turning the safer wrapper into a single
function) so that nobody is tempted to introduce new callers.

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