benchmark

Remove wheel merge job, merge artifacts on download (#1886)

This is supported by `actions/download-artifact@v4`, and endorsed by cibuildwheel
in their documentation (see https://cibuildwheel.pypa.io/en/stable/deliver-to-pypi/#github-actions).

Co-authored-by: dominic <510002+dmah42@users.noreply.github.com>