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

crates/eval/src/example.rs

@@ -283,7 +283,6 @@ impl ExampleContext {
                 | ThreadEvent::ReceivedTextChunk
                 | ThreadEvent::StreamedToolUse { .. }
                 | ThreadEvent::CheckpointChanged
-                | ThreadEvent::UsageUpdated(_)
                 | ThreadEvent::CancelEditing => {
                     tx.try_send(Ok(())).ok();
                     if std::env::var("ZED_EVAL_DEBUG").is_ok() {