Remove staging repo now that packages have been published.

Packages/manifest.json

       "com.unity.xr.arcore": "1.0.0-preview.23",
       "com.unity.xr.arfoundation": "1.0.0-preview.20",
       "com.unity.xr.arkit": "1.0.0-preview.17"
-    },
-    "registry": "https://staging-packages.unity.com"
+    }
   }