Remove wiring for assistant2 (#11940)

This PR removes the wiring for `assistant2` that hooks it up to Zed.

Since we're focusing in on improving the current assistant, we don't
need this present in Zed.

I left the `assistant2` crate intact for now, to make it easier to
reference any code from it.

Release Notes:

- N/A

crates/zed/src/main.rs

@@ -175,7 +175,6 @@ fn init_ui(app_state: Arc<AppState>, cx: &mut AppContext) -> Result<()> {
     inline_completion_registry::init(app_state.client.telemetry().clone(), cx);
 
     assistant::init(app_state.client.clone(), cx);
-    assistant2::init(app_state.client.clone(), cx);
 
     cx.observe_global::<SettingsStore>({
         let languages = app_state.languages.clone();