Clean up unreachable code after tighten DB contraint

Not NULL contraint was applied to namespace_id column on
gitlab_subscriptions table, by merge request
https://gitlab.com/gitlab-org/gitlab/-/merge_requests/54319.
As a result, gitlab_subscription.namespace should always
present. We can safely remove the code where it checks
the `gitlab_subscription.namespace`'s presence.

ee/app/workers/update_max_seats_used_for_gitlab_com_subscriptions_worker.rb

@@ -16,14 +16,6 @@ class UpdateMaxSeatsUsedForGitlabComSubscriptionsWorker # rubocop:disable Scalab
       tuples = []
 
       subscriptions.each do |subscription|
-        # This should be removed after https://gitlab.com/gitlab-org/gitlab/-/issues/243496
-        # Because there should not be any NULL namespace_id gitlab_subscription after the merge
-        # request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/40537
-        unless subscription.namespace
-          track_error(subscription)
-          next
-        end
-
         subscription.refresh_seat_attributes!
 
         tuples << [subscription.id, subscription.max_seats_used, subscription.seats_in_use, subscription.seats_owed]