deploy: Fix removing /var/.updated with separate /var mount
deploy: Fix removing /var/.updated with separate /var mount

There's some subtlety to this, we don't handle all cases.
But the 99% cases are using `--sysroot deploy` to create an
initial deployment, and then doing upgrades from inside
a booted deployment.

It was only the latter case that didn't work with a separate `/var`.
Fixing all of them would probably require libostree to learn
how to e.g. look at `/etc/fstab` (or worse, systemd mount units?)
and handle the mounting.  I don't think we want to do anything
like that right now, since there are no active drivers for the
use case.

Closes: https://github.com/ostreedev/ostree/issues/1729

Closes: #1730
Approved by: akiernan

GNOME/ostree,GNOME/ostree,GNOME/ostree,GNOME/ostree,GNOME/ostree