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

.zed/tasks.json

@@ -3,10 +3,5 @@
     "label": "clippy",
     "command": "cargo",
     "args": ["xtask", "clippy"]
-  },
-  {
-    "label": "assistant2",
-    "command": "cargo",
-    "args": ["run", "-p", "assistant2", "--example", "assistant_example"]
   }
 ]