Remove upgrade CTA for free users from billing page

What does this MR do and why?

Free users who navigate to the /billings page are presented with three "upgrade" CTAs - two (one for the Premium tier and one for the Ultimate tier) direct the user to the SaaS checkout flow. The remaining CTA directs the user to the Customer Portal. Directing a user who does not have a paying relationship with GitLab to the Customer Portal risks creating confusion and potential purchase abandonment for the user. At this point in the user's journey, they have not been introduced to the concept of the Customer Portal, nor its role in maintaining and managing a paying relationship with GitLab.

For this specific user state, we should limit risk for confusion and remove the CTA completely - eliminate confusion with Customer Portal entirely and let user focus on the Upgrade CTAs that will lead to purchase.

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Login as free trial user
2. Go to any group -> /billings and make sure there is no Upgrade button
3. Go to /-/profile/billings and make sure there is no Upgrade button

Related to https://gitlab.com/gitlab-org/gitlab/-/issues/346749