Remove all unused code after Haml version of To-Dos dashboard got retired

Remove the todos_vue_application feature flag and all now unreachable code:

• routes and controller actions
• Haml views and custom CSS
• related Javascript (jQuery! 🔥)
• helpers
• anything on the Todo model?
• any unused finders?
• specs¹
• (anything else?)

¹: Migrate remaining feature specs, if any, to use the new Vue version.

👉 Make sure to leave code that is also used by the Vue version working.