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>