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/assistant2/src/assistant.rs

@@ -34,7 +34,6 @@ pub use crate::inline_assistant::InlineAssistant;
 actions!(
     assistant2,
     [
-        ToggleFocus,
         NewThread,
         NewPromptEditor,
         ToggleContextPicker,