Reduces page_bundle CSS for TODO list

What does this MR do?

Reduces page_bundle CSS for TODO list as well as makes performant in rendering.

LOC of app/assets/stylesheets/page_bundles/todos.scss

before	after
258	94

app/assets/stylesheets/page_bundles/todos.scss cannot be completely eliminated all at once since highlighting a TODO element in a list is not generalized at https://gitlab.com/gitlab-org/gitlab-ui.

Screenshots (strongly suggested)

Case	Before	After
Normal	https://www.youtube.com/watch?v=28VUrcRlpAs&list=PLHs2tISONwl_Mbd1H4z7UmwWWjAxBp-p-	https://www.youtube.com/watch?v=WWSUr0_eWCY
Normal (mobile)	skip	https://www.youtube.com/watch?v=cZL0P66WAGM
on PC for newbie
on mobile for newbie

Does this MR meet the acceptance criteria?

Conformity

• 📋 Does this MR need a changelog?
  • I have included a changelog entry.
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Related to #326918 (closed)