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/assistant.rs

@@ -31,7 +31,6 @@ use crate::slash_command_settings::SlashCommandSettings;
 actions!(
     assistant,
     [
-        ToggleFocus,
         InsertActivePrompt,
         DeployHistory,
         DeployPromptLibrary,