Consolidate Assistant panels in assistant2 feature flag (#23442)

This PR consolidates the two Assistant panels into one for users in the
`assistant2` feature flag.

Now that the Assistant1 prompt editor is accessible through the
Assistant2 panel, we no longer have a need to show both panels.

Release Notes:

- N/A

crates/assistant/src/inline_assistant.rs

@@ -1728,7 +1728,7 @@ impl PromptEditor {
     }
 
     fn placeholder_text(codegen: &Codegen, cx: &WindowContext) -> String {
-        let context_keybinding = text_for_action(&crate::ToggleFocus, cx)
+        let context_keybinding = text_for_action(&zed_actions::assistant::ToggleFocus, cx)
             .map(|keybinding| format!(" • {keybinding} for context"))
             .unwrap_or_default();