Remove in a seat usage quota toggle

What does this MR do and why?

• Removes the 'in a seat' toggle from usage quota page as it is no longer needed based on change in direction
• Removes relating usage quota observation of pending members usage quota page that no longer apply

Screenshots or screen recordings

before	after

How to set up and validate locally

1. Setup GDK to simulate SaaS - https://docs.gitlab.com/ee/development/ee_features.html#simulate-a-saas-instance and restart
2. Enable the invite modal

   Feature.enable(:free_user_cap)

3. Create a Private top-level Group
4. Add 6 members

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 #371662