midx: drop unused parameters from add_midx_to_chain()

midx: drop unused parameters from add_midx_to_chain()

When loading a chained midx, we build up an array of hashes, one per
layer of the chain. But since the chain is also represented by the
linked list of multi_pack_index structs, nobody actually reads this
array. We pass it to add_midx_to_chain(), but the parameters are
completely ignored.

So we can drop those unused parameters. And then we can see that its
sole caller, load_midx_chain_fd_st(), only cares about one layer hash at a
time (for parsing each line and feeding it to the single-layer midx
code). So we can replace the array with a single object_id on the stack.

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