Remove purchase seats button for duo enterprise trial

What does this MR do and why?

For a trial or paid Duo Enterprise:

• It's not possible to self-service add seats for Duo Enterprise. The functionality does not exist in customers portal currently
• If the org is lower than Ultimate tier, they will need to upgrade their tier before they are able to add seats regardless.

So this MR removes the “Purchase seats” button from the GitLab Duo and seat utilization group setting pages, plus adjust the styling of the "Contact sales" button when it's the only button rendered.

Related to #503763

References

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Page	Before	After
/groups/<group_name>/-/settings/gitlab_duo
/groups/<group_name>/-/settings/gitlab_duo/seat_utilization

How to set up and validate locally

1. Setup gdk to simulate SaaS - https://docs.gitlab.com/ee/development/ee_features.html#simulate-a-saas-instance and restart GDK
2. Sign in. Create a new group
3. In rails console create an Ultimate subscription for your new group

namespace = Group.last
gitlab_subscription = namespace.gitlab_subscription # find or create a new one if needed
gitlab_subscription.update(hosted_plan: Plan.find_by(name: 'ultimate'))

4. Apply Duo Enterprise trial to the group

add_on_purchase = GitlabSubscriptions::AddOnPurchase.create(subscription_add_on_id: GitlabSubscriptions::AddOn.find_by(name: 'duo_enterprise').id, namespace: Group.last, started_at: 55.days.ago, expires_on: (Date.today + 5.days), trial: true, quantity: 1, purchase_xid: '1', organization_id: 1)

5. Then you can go to /groups/<group_name>/-/settings/gitlab_duo and /groups/<group_name>/-/settings/gitlab_duo/seat_utilization to verify the changes.