Migrate EDS endpoints_stale_after to xDS TTL

With the introduction of xDS TTL we can now get rid of the EDS specific option `endpoints_stale_after` and instead rely on generic TTL support.

To support this we need to ensure that the EDS subscription code is correctly handling an EDS resource removal, which today is ignored. Once this is done we can go ahead and deprecate the `endpoints_stale_after` field.