Remove incomplete menu-with-popper demo pages

[GavinJoyce]

Summary

Removes placeholder TODO links from the demo site navigation that pointed to unimplemented pages.

Changes

• Remove "Menu with Popper [todo]" link from index page
• Remove "Menu with Popper + Transition [todo]" link from index page
• Delete unused template files: menu-with-popper.hbs and menu-with-transition-and-popper.hbs
• Remove corresponding route definitions from router.js

Testing

• Visit the demo site and verify the menu section only shows "Menu (basic)" and "Menu with Transition" links
• Confirm no broken routes or 404 errors