Remove `force_company_trial` experiment

What does this MR do and why?

The force_company_trial experiment has been marked as a loser and this MR is responsible for removing it.

• The combined_registration experiment will continue to use "force_company_trial" in it's context key as it is still running and there are conversions that we are still possible

part of: #345621 (closed)

Screenshots or screen recordings

These are strongly recommended to assist reviewers and reduce the time to merge your change.

How to set up and validate locally

1. Ensure the combined_registration flow is disabled (Feature.disable(:combined_registration))

2. Sign up for an account (http://localhost:3000/users/sign_in)

3. For my GDK I had to User.last.update(can_create_group: true) although I'm not sure if that's just my machine or not? 😓

4. On the welcome screen (http://localhost:3000/users/sign_up/welcome) choose My company or team under the Who will be using GitLab? option. Submit the form.

5. You should be at this url http://localhost:3000/users/sign_up/groups/new and there should be no trial param. Additionally the trial toggle slider should be "off" or "not active"

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.