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

assets/keymaps/default-macos.json

@@ -207,14 +207,6 @@
       "ctrl-shift-enter": "editor::NewlineBelow"
     }
   },
-  {
-    "context": "AssistantChat > Editor", // Used in the assistant2 crate
-    "bindings": {
-      "enter": ["assistant2::Submit", "Simple"],
-      "cmd-enter": ["assistant2::Submit", "Codebase"],
-      "escape": "assistant2::Cancel"
-    }
-  },
   {
     "context": "AssistantPanel", // Used in the assistant crate, which we're replacing
     "bindings": {