pack-bitmap: drop redundant args from `bitmap_writer_build_type_index()`

pack-bitmap: drop redundant args from `bitmap_writer_build_type_index()`

The previous commit ensures that the bitmap_writer's "to_pack" field is
initialized early on, so the "to_pack" and "index_nr" arguments to
`bitmap_writer_build_type_index()` are redundant.

Drop them and adjust the callers accordingly.

Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>