Remove back-compat code for old agents

              We should also create an issue to remove the old agent case in the release after this one (9.1 I guess?)

_Originally posted by @tevoinea in https://github.com/microsoft/onefuzz/pull/3594#discussion_r1370115135_
            

[AB#169025](https://fuzzfest.visualstudio.com/5eaacf23-2084-4d62-8fbe-7d37326a8b74/_workitems/edit/169025)