Remove runner_list_group_view_vue_ui feature flag

What does this MR do and why?

This change removes the unused code after enabling runner_list_group_view_vue_ui.

Changelog: other

Related to #336405 (closed)

Why is this MR so big?

This MR removes some specific of old the search bar (now replaced by GlFilteredSearch) as well some dangling code at app/controllers/groups/runners_controller.rb. I think it's relevant to remove them here to avoid leaving dead code.

Screenshots or screen recordings

This is the area of the UI that has been removed, and will be deleted in this MR:

How to set up and validate locally

1. As owner of any group
2. Visit the Group -> CI/CD Settings menu
3. Confirm the area only contains a single setting.

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.