* Remove Node Shutdown API feature flag

This PR removes the Node Shutdown API feature flag.

The Node Shutdown API will now always be available.

* Check if xpack is enabled in cleanup

When I removed the feature flag, I assumed that we would always have the
Node Shutdown APIs, but that turns out not to be the case if xpack isn't
enabled. This case was caught by the logic to handle the case where the
feature flag wasn't enabled by accident.

This commit adds the check we always should have had.

* Also check version before tyring cleanup