Remove the existing GitLab Duo Core banners targeting owner/admins

Summary

We will have a new effort to promote awareness and enablement of GitLab Duo Core to existing customers (epic TBC)

Banners to drive awareness and enablement of GitLab Duo Core was initially implemented in https://gitlab.com/gitlab-org/gitlab/-/issues/530958

Solution

To ensure the audience for banners is not seeing the old banners and the new ones, we want to remove the old banners from the code base ahead of the new banners being implemented. This will also allow a bit of a "break" from our in-product banners

Note: The new banners will have new messaging/content but similar display and location criteria https://gitlab.com/gitlab-org/gitlab/-/issues/530958#banner-display-requirements. If there's anything worth saving/documenting with how these banners were initially implemented let's note that