Remove trial from the Learn GitLab template

What does this MR do and why?

Remove trial from the Learn GitLab template

It's because we now use the ultimate template in a custom onboarding screen for both trial and regular onboarding.

• Follow-up from !72355 (merged)
• Related to #343052 (closed)

How to set up and validate locally

Register as a new user in a gitlab.com-like environment (Gitlab.com? = true), and you should see the learn gitlab at the end of the onboarding process.

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.