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/active_thread.rs

@@ -29,8 +29,7 @@ use gpui::{
 };
 use language::{Buffer, Language, LanguageRegistry};
 use language_model::{
-    LanguageModelRequestMessage, LanguageModelToolUseId, MessageContent, RequestUsage, Role,
-    StopReason,
+    LanguageModelRequestMessage, LanguageModelToolUseId, MessageContent, Role, StopReason,
 };
 use markdown::parser::{CodeBlockKind, CodeBlockMetadata};
 use markdown::{HeadingLevelStyles, Markdown, MarkdownElement, MarkdownStyle, ParsedMarkdown};
@@ -72,7 +71,6 @@ pub struct ActiveThread {
     expanded_thinking_segments: HashMap<(MessageId, usize), bool>,
     expanded_code_blocks: HashMap<(MessageId, usize), bool>,
     last_error: Option<ThreadError>,
-    last_usage: Option<RequestUsage>,
     notifications: Vec<WindowHandle<AgentNotification>>,
     copied_code_block_ids: HashSet<(MessageId, usize)>,
     _subscriptions: Vec<Subscription>,
@@ -783,7 +781,6 @@ impl ActiveThread {
             hide_scrollbar_task: None,
             editing_message: None,
             last_error: None,
-            last_usage: None,
             copied_code_block_ids: HashSet::default(),
             notifications: Vec::new(),
             _subscriptions: subscriptions,
@@ -840,10 +837,6 @@ impl ActiveThread {
         self.last_error.take();
     }
 
-    pub fn last_usage(&self) -> Option<RequestUsage> {
-        self.last_usage
-    }
-
     /// Returns the editing message id and the estimated token count in the content
     pub fn editing_message_id(&self) -> Option<(MessageId, usize)> {
         self.editing_message
@@ -949,9 +942,6 @@ impl ActiveThread {
             ThreadEvent::ShowError(error) => {
                 self.last_error = Some(error.clone());
             }
-            ThreadEvent::UsageUpdated(usage) => {
-                self.last_usage = Some(*usage);
-            }
             ThreadEvent::NewRequest | ThreadEvent::CompletionCanceled => {
                 cx.notify();
             }