Remove the assistant2 crate (#12380)

Release Notes:

- N/A

crates/assistant2/src/ui.rs

@@ -1,17 +0,0 @@
-mod active_file_button;
-mod chat_message;
-mod chat_notice;
-mod composer;
-mod project_index_button;
-
-#[cfg(feature = "stories")]
-mod stories;
-
-pub use active_file_button::*;
-pub use chat_message::*;
-pub use chat_notice::*;
-pub use composer::*;
-pub use project_index_button::*;
-
-#[cfg(feature = "stories")]
-pub use stories::*;