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/agent/src/agent_diff.rs

@@ -1366,7 +1366,6 @@ impl AgentDiff {
             }
             // intentionally being exhaustive in case we add a variant we should handle
             ThreadEvent::Stopped(Ok(StopReason::ToolUse))
-            | ThreadEvent::UsageUpdated(_)
             | ThreadEvent::StreamedCompletion
             | ThreadEvent::ReceivedTextChunk
             | ThreadEvent::StreamedAssistantText(_, _)