remount: Drop support for auto-tmpfs-on-var; use systemd.volatile=state

remount: Drop support for auto-tmpfs-on-var; use systemd.volatile=state

In current systemd, there is:
[systemd-volatile-root](https://www.freedesktop.org/software/systemd/man/systemd-volatile-root.service.html)
which was introduced by [this commit](https://github.com/systemd/systemd/commit/91214a37ef4eb8042d2598aa89bae52b410d11a7).

I'd like to make further changes to how we handle `/var`, and I don't
want to reason about the interaction of our "tmpfs var" with too many
other things.

The comment about having "all /var handling in one place" was always inaccurate
given that we rely on systemd for mounting. And in general, I don't want to
duplicate too many things systemd does - it does them well, documents them, etc.

As far as I know, it was basically just Owen who was using this for the GNOME
hardware testing effort, and I'm sure he could easily switch over to
`systemd.volatile=state`.

Closes: #856
Approved by: owtaylor