[XR] Remove staging repo

Packages/manifest.json

     "com.unity.xr.arcore": "1.0.0-preview.20",
     "com.unity.xr.arfoundation": "1.0.0-preview.19",
     "com.unity.xr.arkit": "1.0.0-preview.16"
-  },
-  "registry": "https://staging-packages.unity.com"
+  }
 }