packfile: drop has_pack_index()

The has_pack_index() function has several oddities that may make it
surprising if you are trying to find out if we have a pack with some
$hash:

  - it is not looking for a valid pack that we found while searching
    object directories. It just looks for any pack-$hash.idx file in the
    pack directory.

  - it only looks in the local directory, not any alternates

  - it takes a bare "unsigned char" hash, which we try to avoid these
    days

The only caller it has is in the dumb http code; it wants to know if we
already have the pack idx in question. This can happen if we downloaded
the pack (and generated its index) during a previous fetch.

Before the previous patch ("dumb-http: store downloaded pack idx as
tempfile"), it could also happen if we downloaded the .idx from the
remote but didn't get the matching .pack. But since that patch, we don't
hold on to those .idx files. So there's no need to look for the .idx
file in the filesystem; we can just scan through the packed_git list to
see if we have it.

That lets us simplify the dumb http code a bit, as we know that if we
have the .idx we have the matching .pack already. And it lets us get rid
of this odd function that is unlikely to be needed again.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Taylor Blau <me@ttaylorr.com>

packfile.h

@@ -193,8 +193,6 @@ int find_kept_pack_entry(struct repository *r, const struct object_id *oid, unsi
 int has_object_pack(const struct object_id *oid);
 int has_object_kept_pack(const struct object_id *oid, unsigned flags);
 
-int has_pack_index(const unsigned char *sha1);
-
 /*
  * Return 1 if an object in a promisor packfile is or refers to the given
  * object, 0 otherwise.