Clean up automatic_trial_registration experiment

What does this MR do and why?

Resolves https://gitlab.com/gitlab-org/gitlab/-/issues/417277.

Cleans up the automatic_trial_registration experiment.

The automatic_trial_registration experiment was a success so we have decided to productize it.

Screenshots or screen recordings

URL	Before	After
/users/sign_up => /users/sign_up/company/new

Other Trial Paths with no changes

URL	Before	After
/-/trials/new
/-/trial_registrations/new => /users/sign_up/company/new

How to set up and validate locally

1. Start local GDK simulating SaaS
2. Setup and run CustomerDot
3. Go through register from /users/sign_up
4. On sign_up/welcome page, select My company or team and Create a new project
5. Land on the /sign_up/company form page and observe.
6. Land on Learn GitLab with a trial for the group created during registration.

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 #417277