agent: Remove unneeded tracking of request usage (#29894)

This PR removes some unneeded tracking of the model request usage in the
`ActiveThread` and `ThreadEvent::UsageUpdated` events.

Release Notes:

- N/A
3db4744e18Marshall Bowers committed on 5/5/2025, 1:16:53 AM· committed by GitHubparentfe177f5
4 files changedLine totals unavailable