Experiment Cleanup: `ultimate_trial_benefit_modal`

What does this MR do and why?

Related to #434858

To clean up the ultimate_trial_benefit_modal experiment as the experiment was deemed invalidated. This basically means revert these two MRs:

• !132660 (merged) that introduced the experiment
• !135609 (merged) that added tracking setup in invite modal for the analysis of the experiment

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

Removing Candidate, leaving Control

Control	Candidate

How to set up and validate locally

1. Setup to simulate SaaS and restart GDK.
2. Register as a new user.
3. On the welcome page, select any role and Just me.
4. Chose Create a new project
5. Click Continue
6. Fill out fields to create a group and project on the next page.
7. Click Create project
8. Verify you get to the onboarding and page refreshes to provide a Ok, let's go button.
9. Click the button and verify you are dropped into the learn gitlab page and the control modal pops up with confetti.
10. (You can create a cookie manually in dev tool: confetti_post_signup with value true, and refresh the learn gitlab page to experience it again)