Piuparts job: drop `--warn-on-leftovers-after-purge`

The commit b553c706 added --warn-on-leftovers-after-purge in piuparts. The commit message said (copy-paste relevant part):

    Use piuparts similar to piuparts.debian.org

   - Add --scriptsdir, in particular this fixes sudo installation by setting root password
   - Add --allow-database, to allow DB servers to start
   - Use --warn-on-leftovers-after-purge even if we want a failure, to workaround #316521
...

Currently, piuparts.debian.org is generating the following arguments as seen in https://piuparts.debian.org/sid-strict/fail/simplemonitor_1.14.0a-1.log:

Executing: sudo env PYTHONPATH=/srv/piuparts.debian.org/lib/python3/dist-packages timeout -s INT -k 5m 80m /srv/piuparts.debian.org/bin/piuparts --install-remove-install --scriptsdir /etc/piuparts/scripts --no-eatmydata --allow-database --warn-on-others --scriptsdir /etc/piuparts/scripts-leftovers --mirror 'http://deb.debian.org/debian/ main' --tmpdir /srv/piuparts.debian.org/tmp --arch amd64 -b /srv/piuparts.debian.org/slave/basetgz/sid-merged-usr_amd64.tar.gz -d sid --no-upgrade-test --apt simplemonitor=1.14.0a-1

I think that --warn-on-leftovers-after-purge could be removed from the arguments (https://salsa.debian.org/salsa-ci-team/pipeline/-/blob/master/salsa-ci.yml?ref_type=heads#L902): it can cause the salsa-ci pipeline piuparts job to pass but then might fail in piuparts.debian.org so there is little gain having it.

If someone wants to opt-in: the developer could use the variable SALSA_CI_PIUPARTS_ARGS to add it as documented in !586 (merged)

Are there any steps to make backward-compatible changes? (pipelines that pass now might stop passing... what I do when this happens is to check recent commits in this repository but I don't know if it should be written elsewhere)

I am happy to submit a MR but I wanted to check that this was wanted.