chore: drop com.unity.ugui from demo manifest

The demo is UI-Toolkit-only. No script references UnityEngine.UI,
TMPro, Canvas, Image, Text, Button, GraphicRaycaster, EventSystem,
CanvasScaler, or StandaloneInputModule. No scene contains a Canvas,
CanvasRenderer, or RectTransform component. Dropping the package
also pulls out the TextMeshPro runtime that ships with ugui 2.0 in
Unity 6, and removes the transitive dependency that kept imgui in
the player.

Co-authored-by: Cursor <cursoragent@cursor.com>

Author: fadi-george

examples/demo/Packages/manifest.json

@@ -7,7 +7,6 @@
     "com.unity.inputsystem": "1.19.0",
     "com.unity.nuget.newtonsoft-json": "3.2.2",
     "com.unity.test-framework": "1.6.0",
-    "com.unity.ugui": "2.0.0",
     "com.unity.modules.accessibility": "1.0.0",
     "com.unity.modules.androidjni": "1.0.0",
     "com.unity.modules.jsonserialize": "1.0.0",