boot: Drop ostree-finalize-staged.path

boot: Drop ostree-finalize-staged.path

This effectively reverts ac1a919f ("boot: Add
ostree-finalize-staged.path").

A bug came in on the OCP side that demonstrates that the way things are
setup right now is racy. If a reboot is triggered quickly after staging
a deployment, the whole pipeline of:

- ostree-finalize-staged.path, which triggers
- ostree-finalize-staged.service, which triggers
- ostree-finalize-staged-hold.service,

may not fully have happened before systemd isolates to `reboot.target`
which will want to kill all pending jobs.

Just directly starting the systemd unit is less elegant but much more
explicit and gets rid of any possible race because it's directly part of
the staging operation.

Fixes: https://issues.redhat.com/browse/OCPBUGS-51150