Remove seats_in_use_for_free_or_trial flag

What does this MR do and why?

Remove seats_in_use_for_free_or_trial flag

How to set up and validate locally

This has been turned on for 3 years therefore is stable.

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 #421624 (closed)