pack-bitmap.c: remove unnecessary "open_pack_index()" calls

pack-bitmap.c: remove unnecessary "open_pack_index()" calls

When trying to open a pack bitmap, we call open_pack_bitmap_1() in a
loop, during which it tries to open up the pack index corresponding
with each available pack.

It's likely that we'll end up relying on objects in that pack later
in the process (in which case we're doing the work of opening the
pack index optimistically), but not guaranteed.

For instance, consider a repository with a large number of small
packs, and one large pack with a bitmap. If we see that bitmap pack
last in our loop which calls open_pack_bitmap_1(), the current code
will have opened *all* pack index files in the repository. If the
request can be served out of the bitmapped pack alone, then the time
spent opening these idx files was wasted.S

Since open_pack_bitmap_1() calls is_pack_valid() later on (which in
turns calls open_pack_index() itself), we can just drop the earlier
call altogether.

Signed-off-by: Teng Long <dyroneteng@gmail.com>
Signed-off-by: Taylor Blau <me@ttaylorr.com>