Move UI code from assistant_context_editor -> agent_ui (#33289)

This breaks a transitive dependency of `agent` on UI crates. I've also
found and eliminated some dead code in assistant_context_editor.

Release Notes:

- N/A

crates/zed/src/zed.rs

@@ -9,11 +9,10 @@ mod quick_action_bar;
 #[cfg(target_os = "windows")]
 pub(crate) mod windows_only_instance;
 
-use agent_ui::AgentDiffToolbar;
+use agent_ui::{AgentDiffToolbar, AgentPanelDelegate};
 use anyhow::Context as _;
 pub use app_menus::*;
 use assets::Assets;
-use assistant_context_editor::AgentPanelDelegate;
 use breadcrumbs::Breadcrumbs;
 use client::zed_urls;
 use collections::VecDeque;