Remove runners_finder_all_available feature flag

What does this MR do and why?

This MR removes the runners_finder_all_available which was introduced in this milestone (%15.5). If we manage to merge this within %15.5, we can avoid documenting the FF.

Part of #374525 (closed)

Screenshots or screen recordings

How to set up and validate locally

1. With at least one shared/instance runner registered
2. Visit Admin -> Runners to register one http://gdk.test:3000/admin/runners
3. With at least one group and one runner in your group
4. My Group -> CI/CD -> Runners http://gdk.test:3000/groups/<group>/-/runners
5. Visit My Group -> CI/CD -> Runners
6. Click on "Show only my runners" to enable the full list.

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.