remove support for unshadowed passwords

I'm about to send a mail to debian-devel about this, as I expect it might lead to some debate (update: https://lists.debian.org/debian-devel/2023/01/msg00114.html).

It seems to me that the installer support for unshadowed passwords is past its prime, and ought no longer darken the install of even those souls who've engaged low-priority debconf queries.

1. it appears that /sbin/shadowconfig can be run after install just fine for users of NIS or conserver, or anyone else who doesn't want shadow passwords
2. if someone accidentally selects this, there are very few possible indications down the road that they're running in an unexpected configuration
3. fedora is already discussing removing support for NIS (surely the primary driver of unshadowed passwords) entirely, and it's gone in RHEL
4. it even appears that NIS still works fine with shadowed passwords, or can be made to work fine

popcon shows fewer than 800 installs of the nis package, for whatever that's worth.

send it back to god.

changed the description

merged

mentioned in commit 8e9833c6