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/Cargo.toml

@@ -19,7 +19,6 @@ activity_indicator.workspace = true
 anyhow.workspace = true
 assets.workspace = true
 assistant.workspace = true
-assistant2.workspace = true
 audio.workspace = true
 auto_update.workspace = true
 backtrace = "0.3"