[ILM] Remove the "Remove Policy API" in favour of a settings listener

[colings86]

It may be worth removing the "remove policy API following #34304 and instead registering a Settings listener on the index.lifecycle.name setting which will remove the relevant index metadata from the index when the setting is set to null

[elasticmachine]

Pinging @elastic/es-core-infra

[dakrone]

Tal and I spent some time talking about this. We figured it was a good time to
document some of the issues with doing this without a large change.

The difficulty for this is that when a policy is removed from an index, we want
to remove other various things from the state of the index, so that it returns
to a "clean" state. However, right now there isn't plumbing for invoking a new
cluster state update on a settings change, since the ClusterStateApplier is
meant for updating internal data structures prior to a cluster state update
becoming visible, not for making subsequent changes to the cluster state.

There could be a removal followed by a master node failover causing an election
and the new master would silently not remove the settings.

[dakrone]

This has been open for quite a while, and we haven't made much progress on this due to focus in other areas. For now I'm going to close this as something we aren't planning on implementing. We can re-open it later if needed.