assistant2: Remove unnecessary Pane (#21183)

This PR removes an unnecessary `Pane` that was copied over from
`assistant::AssistantPanel` to `assistant2::AssistantPanel`.

Release Notes:

- N/A

crates/assistant2/src/message_editor.rs

@@ -1,5 +1,5 @@
 use editor::{Editor, EditorElement, EditorStyle};
-use gpui::{AppContext, Model, TextStyle, View};
+use gpui::{AppContext, FocusableView, Model, TextStyle, View};
 use language_model::{
     LanguageModelRegistry, LanguageModelRequest, LanguageModelRequestMessage, MessageContent, Role,
 };
@@ -97,6 +97,12 @@ impl MessageEditor {
     }
 }
 
+impl FocusableView for MessageEditor {
+    fn focus_handle(&self, cx: &AppContext) -> gpui::FocusHandle {
+        self.editor.focus_handle(cx)
+    }
+}
+
 impl Render for MessageEditor {
     fn render(&mut self, cx: &mut ViewContext<Self>) -> impl IntoElement {
         let font_size = TextSize::Default.rems(cx);