deploy: Remove global `sync` by default

deploy: Remove global `sync` by default

Our previous change here was not actually sufficient for
the ceph case, because what (I think) is happening is that
our other `syncfs()` invocation is getting blocked on some
kernel mutexes that are used in `sync`, and that's causing the
process to fully block.

We should not be dependent on a full filesystem `sync`, only
on the sync of the sysroot and boot filesystems.

Anyone who *does* want this behavior could inject an override
for `ostree-finalize-staged.service` that overrides `ExecStop`
to add a run of `sync`.