[Fleet] provide way to automatically unenroll inactive agents.

[n0othing]

Relates to elastic/kibana#143455 + #2063

Today we require users to manually unenroll inactive agents. This can be problematic for environments that spin up thousands of ephemeral agents, creating a massive backlog of inactive agents that need to be unenrolled. While this can be tackled with an external script, it'd be better to provide a way to automate this directly in the application.

[michel-laterman]

I don't think we should implement this; we previously had this as a feature (with the unenroll_timeout setting), but it caused a lot of SDHs from the customers.

See #2063
cc @kpollich

[juliaElastic]

more recent issue to address this: elastic/kibana#179399

[kpollich]

Closing in favor of elastic/kibana#179399