Don't explicitly include labextension in retrolab

It's automatically loaded by retrolab, since it's a labextension!

app/index.js

@@ -88,9 +88,6 @@ async function main() {
     require('@retrolab/docmanager-extension'),
     require('@retrolab/help-extension'),
     require('@retrolab/notebook-extension'),
-    require('@retrolab/lab-extension').default.filter(({ id }) =>
-      ['@retrolab/lab-extension:interface-switcher'].includes(id)
-    ),
     // to handle opening new tabs after creating a new terminal
     require('@retrolab/terminal-extension'),