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/prompt_library/src/prompt_library.rs

@@ -27,7 +27,7 @@ use ui::{
 };
 use util::{ResultExt, TryFutureExt};
 use workspace::Workspace;
-use zed_actions::InlineAssist;
+use zed_actions::assistant::InlineAssist;
 
 pub use crate::prompt_store::*;
 pub use crate::prompts::*;