sponsorship: Remove free_trial from sponsorship page context.

Being on a free trial also means that the billing entity is on a
paid plan.

If they are on a sponsored plan, then they are not on a free trial.
If they have a pending sponsorship request, then they will be
redirected to the billing page. Otherwise, they will be shown the
form to request a sponsored/discounted plan, which does not use
the free_trial boolean.

corporate/lib/stripe.py

@@ -3415,7 +3415,6 @@ class BillingSession(ABC):
             plan = get_current_plan_by_customer(customer)
             if plan is not None:
                 context["plan_name"] = plan.name
-                context["free_trial"] = plan.is_free_trial()
                 context["is_server_on_legacy_plan"] = (
                     plan.tier == CustomerPlan.TIER_SELF_HOSTED_LEGACY
                 )