collab: Remove Zed Free as an option when initiating a checkout session (#31638)

This PR removes Zed Free as an option when initiating a checkout
session, as we manage this plan automatically now.

Release Notes:

- N/A

crates/collab/src/api/billing.rs

@@ -283,7 +283,6 @@ async fn list_billing_subscriptions(
 enum ProductCode {
     ZedPro,
     ZedProTrial,
-    ZedFree,
 }
 
 #[derive(Debug, Deserialize)]
@@ -380,11 +379,6 @@ async fn create_billing_subscription(
                 )
                 .await?
         }
-        ProductCode::ZedFree => {
-            stripe_billing
-                .checkout_with_zed_free(customer_id, &user.github_login, &success_url)
-                .await?
-        }
     };
 
     Ok(Json(CreateBillingSubscriptionResponse {