Remove the billing in side nav experiment

What does this MR do and why?

• Remove billing in side nav experiment
  • mostly a rollback of !74934 (merged)

How to set up and validate locally

1. Turn on billing features in the app(via rails console)

   ApplicationSetting.first.update(check_namespace_plan: true)

2. Visit any group or project pages such as http://localhost:3000/flightjs

3. Observe the Billings is not in the side nav

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 #352177