Remove FF for seat count usage alerts

What does this MR do and why?

Addresses: #362041 (closed)

The FF seat_count_alerts has been enabled globally on production for a month now. There haven't been any relevant errors in Sentry or reported issues, so I think we can remove the FF now.

Screenshots or screen recordings

User count below threshold	User count at threshold	After dismissing alert and page refresh

How to set up and validate locally

Have Gitlab, ZSim and CustomersDot running. The GDK should be running with GITLAB_SIMULATE_SAAS=1

1. Purchase a subscription for a group with three seats
2. Update the namespace statistics
   • UpdateMaxSeatsUsedForGitlabComSubscriptionsWorker.new.perform
3. View the group homepage - the alert should not be displayed
4. Add a developer to the group
5. Update the namespace statistics
   • UpdateMaxSeatsUsedForGitlabComSubscriptionsWorker.new.perform
6. View the group homepage - the alert should be displayed
7. Click the close button on the alert and refresh the page - the alert should not be displayed

MR acceptance checklist

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