Conclude the combined registration experiment

What does this MR do and why?

• conclude the combined_registration experiment as described in #350754 with candidate being the winner
  • removes all control code, which means the groups and projects controllers in registrations are no longer used and therefore removes all code for that and items related to it.
• resolve rubocop todos for combined_registration items
• converts experiment tracking to regular tracking

note

• this feature flag has been fully enabled on .com for months now.
• group/project creation flow only occurs on .com due to this method which is called from show_signup_onboarding? which causes update_success_path to be invoked via this area
• my comments are best viewed from the file(changes tab)

Screenshots or screen recordings

promoted existing combined group/project creation in the registration flow

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

1. Enable https://docs.gitlab.com/ee/development/ee_features.html#simulate-a-saas-instance and restart GDK
2. Register as a new user
3. Select a role
4. Just me
5. Create a new project
6. Observe form/complete it

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 #350754