Remove unused generic CSS classes from new Vue code

What does this MR do and why?

This removes the todo, todo-pending and todo-done classes from the component.

These were not used in the Vue implementation, but only carried over from the old Haml, where they were both used for styling and test selectors.

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

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Before	After

How to set up and validate locally

• Enable the todos_vue_application feature flag.
• Go to http://gdk.test:3000/dashboard/todos/vue and make sure there is no visual or behavioral change between this branch and master.

Related to #483055 (closed)