Remove show_enable_duo_banner feature flag

What does this MR do and why?

Removes the show_enable_duo_banner feature flag while keeping the banner in the settings Gitlab Duo Settings menu.

Banner implemented here: !189055 (merged)

Changelog: other

References

Screenshots or screen recordings

Locally:

How to set up and validate locally

(Taken from implementation MR: !189055 (merged))

1. Make sure your local GDK has a license and not simulating SaaS: https://gitlab.com/gitlab-org/customers-gitlab-com/-/blob/main/doc/setup/gitlab.md#adding-a-license-from-staging-customers-portal-to-your-gdk
2. Sign in as admin
3. Visit /admin and /admin/gitlab_duo to see the banners.

MR acceptance checklist

Evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Related to #541166 (closed)