t/lib-bitmap.sh: avoid silencing stderr

t/lib-bitmap.sh: avoid silencing stderr

The midx_bitmap_partial_tests() function is responsible for setting up a
state where some (but not all) packs in the repository are covered by a
MIDX (and bitmap).

This function has redirected the `git multi-pack-index write --bitmap`'s
stderr to a file "err" since its introduction back in c51f5a6437 (t5326:
test multi-pack bitmap behavior, 2021-08-31).

This was likely a stray change left over from a slightly different
version of this test, since the file "err" is never read after being
written. This leads to confusingly-missing output, especially when the
contents of stderr are important.

Resolve this confusion by avoiding silencing stderr in this case.

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