agent2: Tweak usage callout border (#36777)

Release Notes:

- N/A

crates/agent_ui/src/ui/preview/usage_callouts.rs

@@ -86,10 +86,6 @@ impl RenderOnce for UsageCallout {
             (IconName::Warning, Severity::Warning)
         };
 
-        div()
-            .border_t_1()
-            .border_color(cx.theme().colors().border)
-            .child(
         Callout::new()
             .icon(icon)
             .severity(severity)
@@ -102,7 +98,6 @@ impl RenderOnce for UsageCallout {
                     .on_click(move |_, _, cx| {
                         cx.open_url(&url);
                     }),
-                    ),
             )
             .into_any_element()
     }