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/Cargo.toml

@@ -26,7 +26,6 @@ agent_settings.workspace = true
 anyhow.workspace = true
 askpass.workspace = true
 assets.workspace = true
-assistant_context_editor.workspace = true
 assistant_tool.workspace = true
 assistant_tools.workspace = true
 audio.workspace = true