Remove unused Sidebar Todo component

What does this MR do and why?

Describe in detail what your merge request does and why.

Remove unused Sidebar Todo component

The component seems to have been superceded by ~/sidebar/components/todo_toggle/sidebar_todo_widget.vue. The old epic todo toggle can now be removed.

See also: !68325 (merged)

Screenshots or screen recordings

Screen_Recording_2023-05-02_at_11.43.15

How to set up and validate locally

Toggle the todo on any epic

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.