Remove duplicated Duo Pro alert

What does this MR do and why?

Remove duplicated Duo Pro alert on the billings page if you are on Premium plan

References

Please include cross links to any resources that are relevant to this MR. This will give reviewers and future readers helpful context to give an efficient review of the changes introduced.

MR acceptance checklist

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Screenshots or screen recordings

How to set up and validate locally

• Setup gdk to simulate SaaS - https://docs.gitlab.com/ee/development/ee_features.html#simulate-a-saas-instance and restart GDK
• Sign out. Sign up and create new group during sign up or afterwards.

bin/rails c

GitlabSubscription.last.update(hosted_plan: Plan.find_by(name: 'premium'))

• Visit created group billing page.
• Make sure there is no Introducing the GitLab Duo Pro add-on banner.

Related to #499712