Remove trial_ends_on from namespaces API

This was only used by our customers app. `trial_ends_on` still exists in
the API for `gitlab_subscriptions`, which is correct. The
`namespaces.trial_ends_on` column no longer exists.

ee/lib/ee/api/namespaces.rb

@@ -53,7 +53,6 @@ module EE
           params do
             optional :shared_runners_minutes_limit, type: Integer, desc: "Pipeline minutes quota for this namespace"
             optional :extra_shared_runners_minutes_limit, type: Integer, desc: "Extra pipeline minutes for this namespace"
-            optional :trial_ends_on, type: Date, desc: "Trial expiration date"
           end
           put ':id' do
             authenticated_as_admin!