Stop showing the explore plans link on the billing page

What does this MR do and why?

Stop showing the explore plans link on the billing page

• should not show as we are already on that page
• still show on subgroup billing page though as it does not have the plans

Screenshots or screen recordings

note: the examples below were made by commenting out - return unless show_unlimited_members_during_trial_alert?(root_namespace) in ee/app/views/shared/_unlimited_members_during_trial_alert.html.haml. The billing page content below on the plans will not show this way for trials as it has a different design, but that wasn't important for the before/after

Before	After	After subgroup billing page

How to set up and validate locally

1. Setup to simulate SaaS and restart GDK
2. Comment out - return unless show_unlimited_members_during_trial_alert?(root_namespace) in ee/app/views/shared/_unlimited_members_during_trial_alert.html.haml
3. Navigate to any group you are owner of and go to Settings -> Billings

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #426438 (closed)