Remove /cargo-workspace slash command (#38354)

This PR removes the `/cargo-workspace` slash command.

We never fully shipped this—with it requiring explicit opt-in via a
setting—and it doesn't seem like the feature is needed in an agentic
world.

Release Notes:

- Removed the `/cargo-workspace` slash command.

assets/settings/default.json

@@ -920,14 +920,6 @@
     // Default: 4
     "message_editor_min_lines": 4
   },
-  // The settings for slash commands.
-  "slash_commands": {
-    // Settings for the `/project` slash command.
-    "project": {
-      // Whether `/project` is enabled.
-      "enabled": false
-    }
-  },
   // Whether the screen sharing icon is shown in the os status bar.
   "show_call_status_icon": true,
   // Whether to use language servers to provide code intelligence.