collab: Remove POST /billing/subscriptions/migrate endpoint (#31770)

This PR removes the `POST /billing/subscriptions/migrate` endpoint, as
it is no longer needed.

Release Notes:

- N/A

crates/collab/src/api/billing.rs

@@ -58,10 +58,6 @@ pub fn router() -> Router {
             "/billing/subscriptions/manage",
             post(manage_billing_subscription),
         )
-        .route(
-            "/billing/subscriptions/migrate",
-            post(migrate_to_new_billing),
-        )
         .route(
             "/billing/subscriptions/sync",
             post(sync_billing_subscription),
@@ -632,86 +628,6 @@ async fn manage_billing_subscription(
     }))
 }
 
-#[derive(Debug, Deserialize)]
-struct MigrateToNewBillingBody {
-    github_user_id: i32,
-}
-
-#[derive(Debug, Serialize)]
-struct MigrateToNewBillingResponse {
-    /// The ID of the subscription that was canceled.
-    canceled_subscription_id: Option<String>,
-}
-
-async fn migrate_to_new_billing(
-    Extension(app): Extension<Arc<AppState>>,
-    extract::Json(body): extract::Json<MigrateToNewBillingBody>,
-) -> Result<Json<MigrateToNewBillingResponse>> {
-    let Some(stripe_client) = app.real_stripe_client.clone() else {
-        log::error!("failed to retrieve Stripe client");
-        Err(Error::http(
-            StatusCode::NOT_IMPLEMENTED,
-            "not supported".into(),
-        ))?
-    };
-
-    let user = app
-        .db
-        .get_user_by_github_user_id(body.github_user_id)
-        .await?
-        .context("user not found")?;
-
-    let old_billing_subscriptions_by_user = app
-        .db
-        .get_active_billing_subscriptions(HashSet::from_iter([user.id]))
-        .await?;
-
-    let canceled_subscription_id = if let Some((_billing_customer, billing_subscription)) =
-        old_billing_subscriptions_by_user.get(&user.id)
-    {
-        let stripe_subscription_id = billing_subscription
-            .stripe_subscription_id
-            .parse::<stripe::SubscriptionId>()
-            .context("failed to parse Stripe subscription ID from database")?;
-
-        Subscription::cancel(
-            &stripe_client,
-            &stripe_subscription_id,
-            stripe::CancelSubscription {
-                invoice_now: Some(true),
-                ..Default::default()
-            },
-        )
-        .await?;
-
-        Some(stripe_subscription_id)
-    } else {
-        None
-    };
-
-    let all_feature_flags = app.db.list_feature_flags().await?;
-    let user_feature_flags = app.db.get_user_flags(user.id).await?;
-
-    for feature_flag in ["new-billing", "assistant2"] {
-        let already_in_feature_flag = user_feature_flags.iter().any(|flag| flag == feature_flag);
-        if already_in_feature_flag {
-            continue;
-        }
-
-        let feature_flag = all_feature_flags
-            .iter()
-            .find(|flag| flag.flag == feature_flag)
-            .context("failed to find feature flag: {feature_flag:?}")?;
-
-        app.db.add_user_flag(user.id, feature_flag.id).await?;
-    }
-
-    Ok(Json(MigrateToNewBillingResponse {
-        canceled_subscription_id: canceled_subscription_id
-            .map(|subscription_id| subscription_id.to_string()),
-    }))
-}
-
 #[derive(Debug, Deserialize)]
 struct SyncBillingSubscriptionBody {
     github_user_id: i32,